Youn has enjoyed a great run, with her Oscar triumph coming after wins for Best Supporting Actress in the British Academy of Film and Television Arts (or Bafta) Awards and at the Screen Actors Guild Awards.
In a ceremony marked by socially distanced in-person events in cities around the world connected by a live feed, most of the wins came as no surprise, but there were still a couple of upsets.
McDormand's win came as a shock because before the night, it looked as if she had been left in the shade by fellow nominees.
Carey Mulligan (the thriller Promising Young Woman) had been on a winning streak with critics' awards, while Andra Day (the biopic The United States Vs Billie Holiday) had won the Golden Globe for Best Actress.
Viola Davis (the play-turned-movie Ma Rainey's Black Bottom) had been the favourite because of her Best Actress win in the crucial precursor race, the Screen Actors Guild Awards.
But the biggest shock was reserved for the Best Actor award.
In a break with tradition, the event's producers moved the Best Actor and Best Actress categories to the end of the ceremony, supplanting the event's traditional climax, the Best Picture award.
The producers, including Oscar winner Steven Soderbergh (Traffic, 2000), had expected Black Panther (2018) actor Chadwick Boseman to win posthumously for playing the trumpeter Levee Green in Ma Rainey.
The late Boseman had already won Best Actor at both the Golden Globes and the important Screen Actors Guild Awards.
His win, coming after his death last year from cancer, would have been an emotional moment.
But the evening ended abruptly when it was revealed that Welsh actor Anthony Hopkins won for playing the title role in The Father. At 83, he is the oldest person to receive an Oscar for acting.
Because Hopkins had decided to skip the event, the evening wrapped somewhat anticlimactically.
• An international (edited and shortened) version of the Oscars will air at 11.30am on Channel 5 on Saturday and at 1.45am on Sunday. On Mediacorp's streaming platform meWatch, free on-demand catch-up access to the show will be available until May 19.
RELATED ARTICLE: Soundbites from the Oscars