Colman won best actress for her role in The Favourite and charmed viewers with a funny and tearful speech.
Green Book won three trophies in total, including best picture, which had been expected to go to Netflix's Roma.
Bohemian Rhapsody won the most awards in total with four, while Roma and Black Panther also won three each.
The star, who started out as a sidekick in TV sitcoms like Peep Show, was in shock when her name was called.
Glenn Close had been the firm favourite for her role in The Wife - and now has the unenviable record of seven nominations without a win.
Holding back tears on stage, Colman said: "It's genuinely quite stressful. This is hilarious. I've got an Oscar!"
The first British woman to win the prize since Kate Winslet in 2009, she added: "Any little girl who's practising their speech on the telly - you never know!"
And she endeared herself further to the audience by responding to a signal to end her speech by blowing a raspberry.
The four awards for Bohemian Rhapsody, the authorised biopic of Queen and Freddie Mercury, included best actor for Rami Malek, who won rave reviews for playing the late singer.
"I think about what it would have been like to tell little bubba Rami that one day this might happen to him, and I think his curly-haired little mind would have been blown," he said in his acceptance speech.
"That kid was struggling with his identity, trying to figure himself out, and to anyone struggling and trying to discover their voice - listen, we made a film about a gay man, an immigrant, who lived his life unapologetically himself.
"And the fact I'm celebrating him and this story with you tonight is proof that we're longing for stories like this."
It was later reported that Malek fell off the stage after the ceremony concluded and had to be helped into a seat by security staff.
The 37-year-old was reportedly attended to by paramedics, though he made no reference to the incident later when he spoke to reporters backstage.