2008–2019: Commercial success and stardom as Iron Man
In 2006, Downey was cast as the titular character in Iron Man, a superhero film. Director Jon Favreau explained that although Downey was not the most obvious choice, "he understood what makes the character tick. He found a lot of his own life experience in 'Tony Stark'." Favreau strongly advocated for Downey's casting, believing he could elevate the film's quality and generate widespread audience interest, much like Johnny Depp had done for the Pirates of the Caribbean series. Downey gained more than twenty pounds (nine kilograms) of muscle over five months to achieve the physicality required for the part. Iron Man was released on May 2, 2008, in the US. It became the eighth-highest-grossing film of 2008, earning more than $585 million worldwide against a production budget of $130 million.
In Iron Man, Downey portrayed Tony Stark, a wealthy industrialist who is kidnapped and forced to build a deadly missile, only to instead construct a suit of armor that transforms him into the superhero Iron Man. Both the film and Downey's performance were acclaimed by critics, many of whom considered his portrayal the standout element of Iron Man and credited it with catapulting him to global stardom. Ebert wrote that "it's Robert Downey Jr. who powers the lift-off separating this from most other superhero movies", and, in a retrospective Variety article, Zack Sharf credited Downey's charisma with making Iron Man both a critical and commercial success. By October 2008, Downey had signed on to reprise his role in two sequels to Iron Man and in The Avengers (2012), the latter of which featured the superhero team that Stark joins, based on Marvel's comic book series of the same name. He first reprised the role with a brief appearance as Stark in the film The Incredible Hulk (2008), as part of Marvel Studios' initiative to depict the same Marvel Universe on film by providing continuity among the films.
Downey starred in Tropic Thunder alongside Ben Stiller and Jack Black, the former of whom also directed the film. The three actors portray exaggerated Hollywood archetypes, Downey playing Kirk Lazarus, a self-absorbed, multi–Oscar-winning Australian method actor starring in an overblown Vietnam War film titled Tropic Thunder. To embody his character, African-American platoon sergeant Lincoln Osiris, Lazarus undergoes a controversial skin pigmentation procedure, requiring Downey to wear dark makeup and a wig. Released in August 2008, Tropic Thunder opened atop the American box office and retained the number-one position for three consecutive weeks, eventually grossing $195 million worldwide on a budget of about $90 million. For his portrayal of Lazarus, Downey was nominated for the Academy Award for Best Supporting Actor. He then portrayed Steve Lopez in the biopic The Soloist (2009), a Los Angeles Times columnist who discovers a homeless man playing a violin with virtuoso skill.
Downey played the titular detective in Guy Ritchie's Sherlock Holmes (2009). The Times praised the lead performances, describing Downey as "terrific" and commending Ritchie for eliciting "a career-best performance from [Jude] Law", though it noted that their double act struggled to sustain "an overlong film". Sherlock Holmes became the eighth-highest-grossing film of 2009 and won Downey the Golden Globe Award for Best Actor in a Motion Picture – Musical or Comedy. Downey reprised his role as Stark in Iron Man 2, the first of two planned sequels to Iron Man. Released in May 2010 in the US, the film grossed over $623 million worldwide, ranking as the seventh highest-grossing film of the year. Set six months after Iron Man, the film follows Stark as he rejects government demands to share his technology, while facing the vengeful scientist Ivan Vanko. Ebert called Downey's performance "intriguing" and "unexpected", stating that "he doesn't behave like most superheroes: he lacks the psychic weight and gravitas".
Downey's other film role of 2010 was in Due Date, which was filmed across several locations in the US, including Georgia, New Mexico, Arizona, and California. In it, he played an architect trying to get from Atlanta to Los Angeles in time for his wife's scheduled cesarean section. The film was a box-office success, earning over $211 million on a budget of $65 million. That year, Downey and his wife Susan opened their own production company called Team Downey. Downey reprised his role as Sherlock Holmes in Sherlock Holmes: A Game of Shadows (2011), the sequel to the 2009 film. In A Game of Shadows, he investigates a conspiracy led by his nemesis, Professor Moriarty, and works to prevent an impending world war. Todd McCarthy of The Hollywood Reporter lauded the film's "more mannered shenanigans", whose action-oriented antics "play straight to modern audiences' tastes".
Downey reprised the role of Stark in The Avengers (2012). In the film, the Avengers assemble to stop Loki from conquering Earth using the Tesseract to open a portal for an alien invasion. The film was both a critical and commercial success; grossing over $1.52 billion, it became the third-highest-grossing film of all time. In Iron Man 3 (2013), Stark faces threats from a terrorist known as the Mandarin while dealing with the aftermath of previous events and developing new technology to confront his enemies. Iron Man 3 grossed $1.215 billion, making it the fifth-highest-grossing film of all time upon release. He then portrayed a Chicago lawyer in The Judge (2014)—the first production by Team Downey. He reprised the role as Tony Stark in Avengers: Age of Ultron (2015), in which Stark creates Ultron as a flawed global defense artificial intelligence and then leads the Avengers in their efforts to stop it, ultimately helping to bring Vision to life and defeat Ultron. Age of Ultron garnered $1.405 billion in revenue to become the fifth-highest-grossing film in history.
In Captain America: Civil War (2016), Stark backs government control of the Avengers, recruits Spider-Man, and ends up fighting Captain America and Bucky after discovering the latter killed his parents. Stark continues mentoring Spider-Man in Spider-Man: Homecoming (2017), and the two team up in Avengers: Infinity War (2018) alongside Doctor Strange to fight Thanos on Titan, where Stark devises a plan to remove the Infinity Gauntlet, survives a brutal stabbing, and witnesses half his allies disintegrate in the Snap. He helps invent time travel to retrieve the Infinity Stones in Avengers: Endgame (2019), ultimately sacrificing his life by snapping his fingers to destroy Thanos and his army, saving the universe. Endgame became the first superhero film to gross over $2 billion worldwide, and was the highest-grossing film of all time until it was surpassed by Avatar (2009) due to its 2021 re-release in China.