She beat Amy Adams (Vice), Marina de Tavira (Roma), Emma Stone (The Favourite) and Rachel Weisz (The Favourite).
King described her win as "a little surreal". "James Baldwin birthed this baby" she said, before adding through tears: "I'm an example of what it looks like when support and love is poured into someone."
During an interview with The Independent earlier this month, King spoke about If Beale Street Could Talk not being nominated for Best Picture.
"I’m still in my feelings that Beale Street was not nominated for Best Picture," she said.
The 2019 Oscars saw Rami Malek take home Best Actor for his portrayal of Freddie Mercury in Queen biopic Bohemian Rhapsody. Other winners included Olivia Colman for The Favourite and Alfonso Cuarón took home Best Director for his Netflix drama Roma. Green Book won Best Picture, proving a controversial choice over Roma.
You can find a full list of winners here.
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