The dazzling multiverse adventure won seven awards including best picture, director and original screenplay.
Accepting her statuette, Yeoh said: "For all the little boys and girls who look like me watching tonight, this is a beacon of hope and possibilities.
"And ladies, don't let anybody tell you that you are ever past your prime."
Yeoh's co-stars Ke Huy Quan and Jamie Lee Curtis triumphed in the supporting actor and actress categories. In the history of the Oscars, no other film has ever won best picture, best director and three acting prizes.
In Everything Everywhere All at Once, Yeoh plays a Chinese-American laundrette owner who is mired in a tax audit, stuck in a crumbling marriage and struggling to connect with her daughter Joy.
But when she discovers different versions of herself in the multiverse, she must tap into their skills in order to save the world.
"This is proof that dreams do come true," Yeoh said in her speech. "I have to dedicate this to all the moms in the world because they are the superheroes, and without them, none of us would be here tonight."
The 60-year-old enjoyed a late surge in momentum in this year's Oscars race, ultimately overtaking the early frontrunner Cate Blanchett.
Yeoh is only the second woman of colour to win best leading actress, following Halle Berry for Monster's Ball more than two decades ago.
Best leading actress has historically been far less diverse than the supporting actress category, where Ariana DeBose, Yuh-jung Youn, Regina King, Viola Davis, Lupita Nyong'o and Octavia Spencer have won in the past decade.