Downey played Lewis Strauss, the former chairman of the US Atomic Energy Commission who mounted a behind-the-scenes campaign to strip J Robert Oppenheimer of his security clearance by tarring him as a communist.
Strauss’s efforts were later exposed during a congressional hearing, as he sought confirmation to serve as president Dwight Eisenhower’s commerce secretary.
The actor thanked his “terrible childhood and the Academy – in that order” and his wife Susan, who he said “found me a snarling rescue pet” and “loved me back to life”. He also acknowledged the cast and crew of “Oppenheimer”.
“What we do is meaningful,” Downey said. “What we decide to do is important.”
Critics praised Downey for playing against his conventional type. Though he has played junkies, hustlers and fast-talkers, Downey is perhaps best known for his multiple film appearances as Marvel superhero Iron Man.
Downey was considered a frontrunner for the best supporting actor Oscar, having collected Golden Globe and Screen Actors Guild trophies for his co-starring role.
The actor earned his first Oscar nomination for playing Charlie Chaplin in 1992’s “Chaplin”. After battles with scandal and addiction, he earned a second supporting-actor nomination for his role in the war satire “Tropic Thunder”.
In the ‘Zone’
Meanwhile, Britain’s “The Zone of Interest,” about a German officer’s family living next door to the Auschwitz extermination camp during World War II, won the Oscar for best international feature film.
The film centres on the commandant Rudolf Hoss and his family as they set up a life next to the Auschwitz death camp in occupied Poland, where more than 1.1 million people were murdered in the largest of the concentration camps and extermination centres built by the Nazis.
Accepting the award, director Jonathan Glazer said the film, which explores the capacity for violence in all people and was shot entirely at Auschwitz, is relevant to the global conflicts happening today.
“All our choices were made to reflect and confront us in the present, not to say look what they did then. Rather, look at what we do now,” Glazer said.
“Our film shows where dehumanisation leads, at its worst.”
“The Zone of Interest,” which also won the Grand Prix at the 2023 Cannes Film Festival, is also nominated for best picture and best director Oscars.
Earlier in the proceedings on Sunday, “The Holdovers” star Da’Vine Joy Randolph won the best supporting actress trophy, while “The Boy and the Heron” took home the prize for best animation.