Leonardo Dicaprio beat the likes of British star Eddie Redmayne, Bryan Cranston, Matt Damon and Michael Fassbender to take home the award for his role in The Revenant at the 88th Academy Awards.
The actor was first nominated for an Oscar 23 years ago in a supporting role, and has since been nominated four more times for acting, including this year, but had never won until now.
Leonardo DiCaprio accepts the award for Best Actor for The Revenant
DiCaprio thanked the movie's director and his co-star Tom Hardy, saying "your fierce talent on screen can only be surpassed by your friendship off screen".
Another of the night's coveted gongs, best picture, was awarded to Spotlight, which tells the story of how the Boston Globe newspaper exposed abuse by Roman Catholic clergy.
Room star Brie Larson was named best actress in a leading role and ended her speech thanking "the fans, the moviegoers, thank you for going to the theatre and seeing our films, I appreciate it, thank you".
The Revenant's Alejandro G Inarritu was named best director for the second year in a row, and thanked DiCaprio for "giving your heart, your soul, your life".
Brie Larson accepted the Best Actress award for Room from actor Eddie Redmayne
The most awards of the night went to Mad Max: Fury Road, which managed to take home six Oscars in total, followed by The Revenant with three and Spotlight winning two.
British singer Sam Smith picked up the best original song Oscar and dedicated his award to the lesbian, gay, bisexual, and transgender (LGBT) community.
Smith co-wrote the song Writing's On The Wall with fellow Brit Jimmy Napes for the latest James Bond movie, Spectre. He follows in the footsteps of Adele, who won the Oscar in 2013 for the Bond instalment Skyfall.
Another British star who scooped a surprise victory was Mark Rylance, who was named best supporting actor on a night dominated by the row over the lack of black nominees.
Rylance, who beat off competition from Sylvester Stallone, Mark Ruffalo and fellow Brits Christian Bale and Tom Hardy, won the gong for his role in Bridge Of Spies.
Actor Mark Rylance accepts his award for Best Supporting Actor in Bridge of Spies
Host Chris Rock did not shy away from the diversity issues plaguing this year's awards and opened the ceremony by declaring Hollywood is "racist" and admitting he considered boycotting the ceremony because of the absence of black nominees.
The comedian, who welcomed the audience in the Dolby Theatre to the "white People's Choice Awards", cracked a series of jokes about the race issue and added the controversy over police shootings of black suspects in the US to his targets.
The first British success went to Jenny Beavan, who won best costume design Oscar for Mad Max: Fury Road.
Beavan, who was famously described as a "bag lady" by Stephen Fry at the Baftas, said: "I want to say one quite serious thing, but it could be horribly prophetic, Mad Max, if we are not kinder to each other and if we don't stop polluting our atmosphere."
Costume designer Jenny Beavan accepts the Best Costume Design award for Mad Max: Fury Road
Asked backstage about her choice of Oscars outfit, Beavan said: "I am very happy to talk about it. I don't do frocks and absolutely don't do heels, I have a bad back.
"I look ridiculous in a beautiful gown. This was a homage to Mad Max and I obviously didn't get it quite right at the Baftas, the scarf was meant to be an oil rag."
Alan Rickman, Christopher Lee and David Bowie were among the stars remembered in the 'In Memoriam' section.
American vice president Joe Biden appeared on stage to introduce Lady Gaga ahead of her performance of her song Til It Happens To You, which she co-wrote with Diane Warren for the documentary The Hunting Ground.
