In the film, King portrays Sharon, mother to Tish (Kiki Layne), a young woman who has learned she is pregnant just before her boyfriend is wrongly accused of a terrible crime.
Speaking through tears to a crowd that included fellow nominees like Emma Stone, Rachel Weisz, Marina de Tavira and Amy Adams, King used her platform to honour Beale Street author James Baldwin and director Jenkins.
"To be standing here, representing one of the greatest artists of our time, James Baldwin, is a little surreal," the actress, who also took home a Golden Globe this year, said. "James Baldwin birthed this baby, and Barry, you nurtured her, you surrounded her with so much love and support. So it's appropriate for me to be standing here because I am an example of what happens when support and love is poured into someone."
King also added a special shout-out to her mother, whom she took to the award ceremony.
"Mom, I love you so much," King said. "Thank you for teaching me that God is always leaning, always has been leaning in my direction."
She also added a nod to the women who did not take home the Best Supporting Actress award this year.
"My sisters in art, Marina, Amy, Emma, Rachel, it has been an honour for my name to be said with yours," King added, as the camera revealed a moved Stone with tears in her eyes. "This whole step of the way has been pretty amazing."
The audience was thrilled by King's win — and so was Twitter.
King is reportedly only the second Best Supporting Actress winner to not have received a nod for a SAG or BAFTA award for the role.