Culkin, who started acting as a child, was honoured for playing one of two cousins who travel to Poland to study their family’s history.
“I have no idea how I got here,” Culkin said while accepting his award. “I’ve been acting all my life. I never felt like this was my trajectory.”
A star of the HBO TV series “Succession”, Culkin was a heavy favourite to win after sweeping Bafta, Critic’s Choice, Golden Globe and SAG awards for the film, which was also nominated for best original screenplay this year but missed out on picture and director nods.
Meanwhile, independent film “Flow” won the best animated feature film Oscar, securing the first Academy Award for Latvia and its Latvian director Gints Zilbalodis.
“Flow” follows a cat that finds refuge on a boat along with other animals that join together after a flood destroys their homes.
The movie stood out last year, having been rendered on a free and open-source software platform called Blender and having no dialogue.
Nominees Ariana Grande and Cynthia Erivo opened the night at the Dolby Theatre in Hollywood with a “Wizard of Oz”-themed medley including the showstopping hit “Defying Gravity” from their film “Wicked”.
Grande, Zoe Saldana, Selena Gomez and other stars wore shiny and sculptural gowns. Timothee Chalamet chose a canary yellow tuxedo, prompting a jab from host Conan O’Brien, who said: “You will not get hit on your bike tonight.”
O’Brien also threatened any winners who spoke too long that he would show their old headshots or cut to a shot of actor John Lithgow “looking slightly disappointed”.
Any of three films could score best picture, according to Oscars pundits. One is “Anora”, the story of a sex worker with a shot at a Cinderella story. The other two are “The Brutalist”, about a Jewish immigrant and architect chasing the American dream, and “Conclave”, which imagines the secret proceedings for choosing a pope.
Others in the best picture field include blockbuster musical “Wicked”, and “A Complete Unknown”, the Bob Dylan biopic starring Chalamet.
Netflix musical “Emilia Perez” heads into the ceremony with the most nominations. But its chances of victory dwindled when offensive social media posts surfaced from star Karla Sofia Gascon.
The actress, the first openly transgender person nominated for an acting Oscar, disappeared from the awards circuit but attended Sunday’s ceremony.