After eight Oscar nominations, it was a story about his hometown which he left aged nine that won Kenneth Branagh his first Academy Award.
Branagh's Belfast won Best Original Screenplay, something he said was an "enormous honour".
Accepting the award, he paid tribute to an "amazing city and a fantastic people".
"This story is a search for joy and hope in the face of violence and loss," he said.
"We will never forget all of those lost in the heart-breaking, heart-warming human story of that amazing city of Belfast on the fabulous island of Ireland.
"This means a lot."
Branagh's attendance was only confirmed on Sunday morning after he tested positive for Covid earlier this month.
Ciarán Hinds' attendance was also in doubt after a positive Covid test, but he was able to attend after testing negative.
The win came after a disappointing start to the ceremony for Belfast and its seven nominations, with the first three going elsewhere.
The first award out of the gate was for Best Supporting Actress in which Dame Judi Dench was nominated.
The winner was West Side Story's Ariana DeBose. Next up was Best Sound and Belfast lost out to sci-fi drama Dune.
North Belfast actor Ciarán Hinds lost out to Coda's Troy Kotsur, but before the ceremony he spoke of his joy for being a part of Belfast.
"I don't know how I deserved it, or why I deserve it," he said, on the red carpet.
"I haven't sold my soul to the devil. To be honoured to have Ken ask me to play his grandfather, then to have Judi Dench, the great dame, as my wife was a thrill beyond compare."
Hinds said he wasn't surprised people had taken the film to their hearts.
"It's like Coda as well, they're very human stories, it's about family and difficulties.
"There is so much heart and love involved, so when people go into this world, and it's not their world, Belfast is not their world, but when they get inside the family, it starts to touch them very deeply.
"That's thrilling to see."
'An electric atmosphere'
Catherine Morrison, BBC News NI, in Los Angeles
It was clear how much the award meant to Sir Kenneth Branagh.
When he went up to accept, he thanked the people of Belfast, the "fabulous" island of Ireland and said it was a wonderful tribute to his family.
He only wrote the story during lockdown so it was a pretty quick turnaround from there to the red carpet.
He said those lost in the Troubles would never be forgotten in the story of Belfast.
The atmosphere on the red carpet was electric - there was a real feeling that this was it - the Oscars was back for the first time since the pandemic as electric.
It was strange then that this Oscar ceremony may mostly be remembered for Will Smith hitting Chris Rock in the face on stage.
At the start it wasn't clear if they were doing a bit or some kind of a skit.
Then it soon become clear it wasn't.
One thing is clear - the stars being interviewed afterwards don't seem keen to weigh in with their opinions.
Before the ceremony, Dench recalled Branagh reading her the script for Belfast.
"It was very emotional because it is his childhood story of Belfast," she said.
"We know each other very, very well and he just went straight through without a pause. I can't read anymore now because my eyesight is so terrible, so I felt incredibly privileged that he came and did it."
Bond theme No Time to Die won Best Song over Van Morrison's Down to Joy.
Branagh was clearly moved by his first Oscar win, but couldn't duplicate that success in the Best Director or Best Picture categories.
Jane Campion won Best Director for The Power of the Dog and Coda won Best Picture.
The film, written by Branagh in the first coronavirus lockdown of 2020 and filmed in 2021, has made $50m (£38m) at the theatrical box office.
Branagh said it was "beautiful to see a Belfast story speak to the world".
Richard Williams, the chief executive of Northern Ireland Screen, said the film's global success had given the Northern Ireland film industry "a much-needed boost after a tough two years".
"Having a film named after our capital city recognised at the Oscars gives our industry credibility you couldn't get any other way," he added.