Manchester United
In October 2020, Garnacho joined the Manchester United Academy. United paid Atlético a £420,000 fee. He signed his first professional contract with the club in July 2021. Garnacho came under the spotlight for his solo goal in the FA Youth Cup win over Everton, which was nominated for United's Goal of the Month award for February 2022. After being named as an unused substitute for several Premier League matches, he made his first-team debut for United on 28 April, replacing Anthony Elanga in the 91st minute of a 1–1 draw against Chelsea. Garnacho won the Jimmy Murphy Young Player of the Year award in May. He scored twice in the final of the FA Youth Cup against Nottingham Forest on 11 May, helping United win the competition for the first time since 2011.
For the start of the 2022–23 Premier League season, he changed his shirt number from 75 to 49. On 4 October, he scored a late winner for United's U-21 against Barrow in the 2022–23 EFL Trophy. Garnacho made his first senior start for United on 27 October, in a 3–0 win over Moldovan team Sheriff Tiraspol in the UEFA Europa League, whereafter he was praised by manager Erik ten Hag for his improvement over the past few weeks, saying that though previously dissatisfied, he was happy with Garnacho's improved attitude and resilience. On 3 November, he scored his first senior goal in a Europa League match against Real Sociedad with an assist from his idol Cristiano Ronaldo, and scored his first goal in the Premier League on 13 November, an injury-time winner against Fulham. On 14 January 2023 in the Manchester derby, Garnacho set up Marcus Rashford's 82nd-minute winner in a 2–1 win against Manchester City. On 14 March, Garnacho announced he would be sidelined for several weeks after suffering an ankle ligament injury during a game against Southampton. On 28 April, Garnacho signed a new contract extension that will last until 30 June 2028. On 13 May, Garnacho returned from two months out with injury, scoring the second goal in a 2–0 victory against Wolverhampton Wanderers.
At the beginning of the 2023–24 Premier League season, United announced that Garnacho's squad number would be changed from 49 to 17. On 26 November, Garnacho scored his first league goal of the season with a bicycle kick from 15 yards out to open the scoring in a 3–0 win over Everton, earning him the Premier League Goal of the Month for November 2023, and eventually the Premier League Goal of the Season and FIFA Puskas Award. Three days later, he scored his first Champions League goal in a 3–3 away draw against Galatasaray. On 26 December, Garnacho scored two goals in a 3–2 comeback win against Aston Villa. On 4 February 2024, he scored another brace in a 3–0 win over West Ham United. On 4 April, he scored twice once again, this time in a 4–3 away defeat to Chelsea, becoming the first teenager to score braces in three Premier League matches in a single season since Michael Owen in 1998–99. In the FA Cup final on 25 May, Garnacho scored the opening goal in an eventual 2–1 win over Manchester City. His goal in the final was the first for a teenager since Cristiano Ronaldo in 2004, and he also became the second Argentinian to score after Ricardo Villa's brace for Tottenham Hotspur also against City in 1981.
On 10 August, Garnacho opened the scoring against Manchester City in the FA Community Shield. The match eventually ended in a 1–1 draw and went to penalties, and although Garnacho converted his penalty, United lost the shoot-out 7–6. On 17 September, he scored two goals and provided two assists in United's 7–0 rout of Barnsley in the third round of the EFL Cup. On 27 October, Garnacho made his 100th appearance for United in a 2–1 defeat to West Ham at London Stadium; manager Erik ten Hag was sacked less than 24 hours after the result, with assistant Ruud van Nistelrooy taking charge on an interim basis. Prior to a Europa League match against PAOK on 7 November, a clip emerged on social media of a fan verbally abusing Garnacho while signing autographs. Three days later, in Van Nistelrooy's final match as interim, he scored United's third goal in their 3–0 win over Leicester City. He did not celebrate the goal as, according to United captain Bruno Fernandes, he felt he had "lost faith" from some supporters. New head coach Ruben Amorim dropped Garnacho alongside Marcus Rashford from the squad completely for the Manchester derby on 15 December, which United won 2–1.
