Cesare Fiorio, the sports director, gave Alonso a test in a Formula One (F1) car at the Circuito de Jerez in December 1999, as part of the Euro Open by Nissan's organising company RPM's agreement to give its series champion an opportunity to test at a higher level. He was Minardi's test and reserve driver in 2000 before joining its race team in 2001. In a non-competitive car, Alonso's best result of the season was a tenth-place finish in the German Grand Prix and scored no points for 23rd overall.
Alonso signed as Renault's test driver for 2002 per the orders of manager Flavio Briatore to familiarise himself with the team and improve himself for the future. Alonso worked with the engineering department to improve Giancarlo Fisichella's and Jenson Button's performance, and tested in Spain and the United Kingdom. He drove a Jaguar in an evaluation session against test drivers André Lotterer and James Courtney at the Silverstone Circuit in May 2002. Alonso was promoted to the Renault race team for 2003. He went on to break the records of youngest driver to win a pole position at the season's second race, the Malaysian Grand Prix, and broke Bruce McLaren's record as the youngest F1 race winner at the Hungarian Grand Prix later in the year. He achieved four podium finishes in 2003 and was sixth in the World Drivers' Championship with 55 points.
Alonso remained with Renault for 2004. Alonso had an improved season: he finished the season-opening Australian Grand Prix in third position and took three more podium finishes that year. He took pole position for the French Grand Prix but achieved no race victories en route to fourth in the World Drivers' Championship with 59 points. Alonso stayed at Renault for 2005. He duelled with McLaren driver Kimi Räikkönen for the World Championship in 2005 due to regulation changes prohibiting teams from changing tyres during a race and requiring engines to last for two races before they could be changed. Alonso's car was more reliable than Räikkönen's albeit lacking in speed. Alonso eclipsed Emerson Fittipaldi as the youngest World Drivers' Champion, having won seven victories, six pole positions and fourteen podium finishes for 133 points altogether.
Alonso signed a contract extension with Renault for 2006 in April 2005. Bookmakers installed Alonso as the favourite to retain the Drivers' Championship. His primary competition was Ferrari driver Michael Schumacher. Alonso won six of the first nine races and finished no lower than second to lead the championship with 84 out of a possible 90 points. An Fédération Internationale de l'Automobile (FIA; F1's governing body)-imposed ban on Renault's tuned mass damper device to slow Alonso and an increase of development into Schumacher's Ferrari for competitiveness saw the two tied on points entering the season's penultimate round, the Japanese Grand Prix. Alonso won the race as Schumacher retired due to an engine failure whilst leading. He needed to score one point at the season-ending Brazilian Grand Prix for a second title. Alonso won the championship by finishing second and was Formula 1's youngest double World Champion.
Alonso and McLaren team owner Ron Dennis met secretly in Japan after Dennis talked to Alonso about driving for the team in the future and Alonso expressed interest in the idea. Both men agreed to a three-year contract for Alonso to drive for McLaren starting from 2007. Alonso's contract with Renault expired on 31 December 2006, and he was not granted an early release for sponsorship reasons. Renault allowed Alonso to make his first appearance for McLaren in a test session at the Circuito de Jerez in November 2006. His main competitors in 2007 were his teammate Lewis Hamilton and Räikkönen at Ferrari. Alonso achieved four Grand Prix victories in Malaysia, Monaco, Europe and Italy and led the championship until Hamilton overtook him. Prior to the season's final round, the Brazilian Grand Prix, he had 103 championship points to Räikkönen's 100 and Hamilton's 107, and needed to win the race and for his teammate to finish third or lower for his third title. Alonso finished the event third for third overall with 109 points. He had the same number of points as Hamilton; the tie was broken on count-back as Hamilton finished second more often than Alonso.
Throughout the season, Alonso and Hamilton were involved in a number of incidents, such as the espionage scandal and the flare-up during qualifying for the Hungarian Grand Prix when Hamilton disobeyed a team instruction, thus disadvantaging Alonso, and Alonso responded by delaying Hamilton in the pit lane. The tensions culminated in Alonso and McLaren terminating their contract by mutual consent in November. Alonso was forbidden from joining a team whom McLaren considered their primary challengers for 2008. After rejecting offers from several teams, he signed a two-year contract to rejoin Renault from 2008 because of the manufacturer's long-term commitment to F1 and on-track record. Alonso's car lacked power early on due to an imposed moratorium in development and he scored nine points in the first seven races. He was thereafter able to improve his performance later due to aerodynamic developments to the car's and won in Singapore and Japan; the former race saw Renault order his teammate Nelson Piquet Jr. to crash deliberately and trigger the deployment of the safety car in what became known as "crashgate". Alonso scored more points than any other driver in the final five races with 43. He scored 61 points for fifth in the Drivers' Championship.
Alonso was due to become a free agent for 2009 if Renault were lower than third in the Constructors' Championship. After offers from Red Bull Racing and Honda, he re-signed to Renault on a two-year contract. His car proved to be noncompetitive because it lacked a dual diffuser system and outright speed. Alonso eschewed an aerodynamic front wing mandated in an attempt to make overtaking more possible since he did not believe it would help him. He scored points in eight races and achieved one podium finish: a third place at the Singapore Grand Prix. Alonso won pole position for the Hungarian Grand Prix and led the first twelve laps before he retired following an incorrectly fitted right-front wheel. Alonso was ninth in the Drivers' Championship with 26 points, his lowest placing since he came sixth in 2003; he maintained his reputation as one of F1's best drivers.
Alonso agreed with Ferrari president Luca Cordero di Montezemolo to drive for Ferrari in 2009, but team principal Jean Todt extended the contracts of both Felipe Massa and Räikkönen to 2010. Alonso obtained a mid-2009 agreement to drive for Ferrari from 2011 on but it was moved to 2010 after Renault were investigated for race fixing in Singapore and Räikkönen was released from the team. McLaren's Hamilton and Button and Red Bull's of Sebastian Vettel and Mark Webber were Alonso's main championship competition. He won five races that season and entered the season-ending Abu Dhabi Grand Prix leading by eight points after being 47 behind mid-season following errors. Alonso finished runner-up to Vettel after finishing seventh thereby losing nineteen points to Vettel who won the race.
Alonso's 2011 season was mixed: his car was built conservatively and lacked aerodynamic grip and tyre handling in qualifying. He extracted additional pace from his car to claim ten podium finishes and win the British Grand Prix after a strategy error from Red Bull. His best qualification of the year was a second at the Canadian Grand Prix and he out-qualified his teammate Massa fifteen times over the course of the season. Alonso was fourth overall with 257 points; he was in contention to finish second to eventual champion Vettel following a series of strong finishes until Webber won the season-ending Brazilian Grand Prix.
Ahead of 2012, Alonso extended his contract with Ferrari until 2016. His main competition for the title in 2012 was Vettel. Wins in Malaysia, Valencia and Germany and consistent points-scoring finishes allowed him to build a 40-point lead in the Drivers' Championship. Thereafter start-line collisions, a mechanical failure and an improved performance for Vettel eliminated Alonso's points lead. Alonso entered the season-ending Brazilian Grand Prix 13 points behind Vettel and needed to finish third and for Vettel not to score points for a third championship. He was second and Vettel finished fourth, despite spinning on the opening lap, resigning Alonso to be runner-up for the second time in his career on 278 points.
To begin 2013, Alonso drove an aggressively designed car allowing him to win in China and Spain and consistently scored points. He was slower than Vettel after a change of tyre compound at the German Grand Prix and front and rear bodywork components intended to improve his car's performance were ineffective. With 242 points, Alonso was second for the third time in his career. His relationship with Ferrari cooled due to his perception the team could not construct a title-winning car.
Alonso's 2014 season saw him achieve no race wins because his car was less powerful than the championship-winning Mercedes but took third in the Chinese Grand Prix and second in the Hungarian Grand Prix. Alonso fell to sixth in the Drivers' Championship with 161 points. He qualified faster than his teammate Räikkönen sixteen times by an average of more than 1⁄2 second per lap in 2014.
Alonso had severe disagreements with team principal Marco Mattiacci in 2014 and left Ferrari after contract negotiations to remain at the team fell through. He rejoined McLaren on a three-year contract from 2015 to 2017 with no opt-out clauses. An accident during a pre-season test session at Spain's Circuit de Barcelona-Catalunya in February 2015 saw Alonso sustain a concussion and he was replaced by reserve driver Kevin Magnussen for the season-opening Australian Grand Prix. He endured a difficult season: his car's Honda engine was under-powered and overall speed leaving him vulnerable to being passed. Alonso scored points twice in 2015: a tenth in the British Grand Prix and a fifth in the Hungarian Grand Prix for seventeenth in the Drivers' Championship with eleven points. He was dissatisfied with a slow pace, which became evident after multiple radio complaints that year.
Despite the unreliable and noncompetitive car, Alonso remained with McLaren for 2016. Injuries from a heavy crash with Esteban Gutiérrez at the season-opening Australian Grand Prix caused him to miss the Bahrain Grand Prix on medical grounds and was replaced by reserve driver Stoffel Vandoorne. He qualified better than teammate Button fifteen times and scored points nine times, which included two fifth-place finishes in the Monaco Grand Prix and the United States Grand Prix. He was tenth in the Drivers' Championship with 54 points.
Alonso stayed at McLaren in 2017, but poor reliability affected his season, particularly during the early rounds, and his best finish was a sixth place in the Hungarian Grand Prix. After three consecutive top-ten finishes, Alonso finished fifteenth in the Drivers' Championship with seventeen points.
Following contract negotiations with the McLaren CEO Zak Brown, Alonso signed a multi-year extension with McLaren on 19 October 2017. He finished fifth at the season-opening 2018 Australian Grand Prix and took nine top-ten finishes. Alonso out-qualified his teammate Stoffel Vandoorne at every race and drove quickly and aggressively. He became increasingly annoyed with certain drivers and his commitment to F1 waned after McLaren stopped developing their car to focus on 2019. Alonso was eleventh in the Drivers' Championship with fifty points, and left the sport as a driver at the end of the 2018 season, citing a perceived lack of on-track racing, the predictability of results and felt discussions away from racing about the broadcast of radio transmissions and polemics harmed the series.
Alonso remained at McLaren as a brand ambassador to aid and advise drivers and drove in select test sessions to develop their cars. Alonso drove the MCL34 during a two-day in-season post-race Bahrain test in April 2019 to develop tyres for Pirelli. No further runs were planned for him and McLaren focused on their current drivers. Alonso's ambassador contract with McLaren expired at the end of 2019, and was not renewed for 2020.
Alonso was signed to drive for Alpine F1 Team for the 2021 season, alongside Esteban Ocon, with Renault having rebranded the team under its new name. In preparation for his F1 return, Alonso performed four testing days driving the Renault R.S.18 and was quickest in the post-2020 season young driver's test driving the Renault R.S.20 for Renault. In his first race with Alpine at the 2021 Bahrain Grand Prix, Alonso was forced to retire after plastic debris entered his brake duct. At the 2021 Emilia Romagna Grand Prix he finished in eleventh after qualifying fifteenth, with teammate Ocon finishing ahead in tenth, but both were upgraded one position after Kimi Räikkönen was penalised, giving Alonso his first points of the season.
In Hungary, Alonso temporarily led the race before he made a pit stop and fell to fourth, ahead of Lewis Hamilton. Teammate Ocon credited Alonso's defence against Hamilton with enabling him to achieve his first race victory. In August 2021, Alonso invoked an option to extend his contract for the 2022 season. Alonso scored points in multiple races following the summer break, finishing sixth in the Netherlands, eighth in Italy, sixth in Russia, having run in third in Russia before being forced to pit under wet conditions, and third in Qatar. His third place finish at Qatar was his first podium finish since the 2014 Hungarian Grand Prix.
For the 2022 season, Alonso remained with Alpine. Alonso achieved his highest start driving for Alpine during wet qualifying for the Canadian Grand Prix, starting in second, but had to endure questionable strategies and an engine issue that developed during the race. He dropped down to seventh, and furthermore, received a post-race time penalty that dropped him down to ninth.
Alonso joined Aston Martin on a multi-year deal in 2023 alongside Lance Stroll. He joined the team because he wanted a multi-year contract extension, and Alpine was only willing to give him one more year in F1.
On his Aston Martin debut at the 2023 Bahrain Grand Prix, Alonso, benefitting from the much-improved machinery of the Aston Martin AMR23, recovered from a first-lap contact with his teammate Lance Stroll without any damage and went on to finish in third place, securing a podium finish and Aston's first since Sebastian Vettel's podium at the 2021 Azerbaijan Grand Prix (the German would finish second at the 2021 Hungarian Grand Prix but would be disqualified due to insufficient fuel sample). Despite having to serve a penalty due to his car being off-position at the starting grid, he finished in third again at the following race at the 2023 Saudi Arabian Grand Prix; this marked his 100th podium, making him the sixth driver to have scored one-hundred podiums in his career. Following the race, he was issued another ten-second penalty due to serving the first one improperly at his pit stop, dropping him to fourth behind George Russell; however, the team's appeal was accepted and the second penalty was reversed, keeping his podium.
Alonso finished in third once again after a chaotic Australian Grand Prix; the third and final restart of the session saw his car make contact with the Ferrari of Carlos Sainz Jr., causing the latter driver to receive a five-second penalty; due to this, Sainz would be classified in last place out of the finishing cars. Alonso's car was undamaged. Alonso agreed with Sainz's criticisms of the penalty, stating that it was "too harsh". The Aston Martins were plagued with DRS issues during qualification of the Azerbaijan Grand Prix, going on to qualify eighth in the new "sprint shootout" qualification and sixth for the main qualification session; he would finish sixth in the sprint race, and fourth in the main race. It was at this point in time the car had dropped in performance, failing to secure podiums on a consistent basis, though he took two podium finishes at Zandvoort, where he recorded his first fastest lap since the 2017 Hungarian Grand Prix, and São Paulo; the latter instance edging out Sergio Pérez by 0.053 seconds. After a seventh-place finish at the Abu Dhabi Grand Prix, Alonso ended the season fourth in the standings, scoring 206 points against his teammate Stroll's 74. Alonso's fourth position in the standings was his highest finish since 2013.
Alonso remained with Aston Martin for the 2024 season. He secured his best finish of the season at the 2024 Saudi Arabian Grand Prix, where he qualified fourth and finished fifth. At the following race in Australia, he was issued a 20-second post-race penalty for potentially dangerous driving while defending against George Russell on the penultimate lap. The stewards ruled that Alonso had lifted and braked unusually early into Turn 6 causing a massive closing speed that caused Russell to lose control and crash. The penalty dropped Alonso to eighth. At the Chinese Grand Prix Alonso retired from the sprint race following a puncture from contact with Carlos Sainz. Stewards again deemed Alonso at fault, handing him a ten-second penalty and three penalty points that brought his total to six within a single month. At the 2024 Mexico City Grand Prix, Alonso competed in his four-hundredth Grand Prix weekend, becoming the first driver to reach this number of races. Alonso finished the season 9th in the standings, scoring 70 points against Stroll's 24.
On his third season with Aston Martin at the 2025 Australian Grand Prix, Alonso’s campaign began poorly as the AMR25 struggled for pace. Alonso retired after crashing on lap 34 while his teammate Lance Stroll secured a sixth-place finish. This difficult start persisted through the early rounds of the season where the car lacked the performance of its predecessor, leaving Alonso scoreless and sitting 17th in the standings by May. A significant upgrade package introduced at the Emilia Romagna Grand Prix began to turn his season around as Alonso qualified 5th but finished 11th courtesy of a poorly-timed virtual safety car. At the Hungarian Grand Prix, he finished in fifth place, his highest result of the 2025 campaign. This momentum carried into the second half of the season, where he scored in most rounds from Zandvoort to Abu Dhabi. In both Singapore and Qatar Alonso finished in seventh. Despite the car’s early-season deficit Alonso’s pace allowed him to achieve a 24–0 qualifying clean sweep over Stroll, maintaining a gap of nearly four-tenths of a second on average. After a P7 finish at the 2025 Abu Dhabi Grand Prix, Alonso ended the 2025 season 10th in the Drivers' Championship with 56 points compared to Stroll’s 33.
Alonso began his 2026 campaign with two retirements at Australia and China, before placing 18th in Japan, the team's first finish of the season. The AMR26, the first Aston Martin and only car on the grid to be powered by a Honda engine, suffered from reliability issues and lacked pace, causing a very poor start to the season. Alonso was able to score Aston Martin's first points of the season at the 2026 Monaco Grand Prix, after Nico Hülkenberg and Sergio Pérez both received a ten-second penalty, which resulted in Alonso's promotion to tenth place. Alonso scored two further points with a ninth place finish at the 2026 Dutch Grand Prix.
