The US star was found dead at his home in Brooklyn, New York earlier this month, at the age of 54.
Following an examination, it has been determined that Michael died of an accidental overdose, after taking heroin, cocaine, fentanyl and p-fluorofentanyl.
Michael’s most prolific role was the hitman Omar Little in The Wire, as well as Albert “Chalky” White in the drama Boardwalk Empire.
Throughout his career, he earned four Emmy nominations for his performances in Bessie, The Night Of, When They See Us and, most recently, Lovecraft Country.
His big-screen credits include the films 12 Years A Slave, RoboCop, Inherent Vice and Motherless Brooklyn.
Following the news of his death, Michael was remembered in tributes by stars like Idris Elba, Mariah Carey, Spike Lee and Viola Davis.
The Wire creator David Simon tweeted: “Too gutted right now to say all that ought to be said.
“Michael was a fine man and a rare talent and on our journey together he always deserved the best words. And today those words won’t come.”
A statement from HBO – the US network which was home to both The Wire and Boardwalk Empire – said: “We are devastated to learn of the passing of Michael Kenneth Williams, a member of the HBO family for more than 20 years.
“While the world is aware of his immense talents as an artist, we knew Michael as a dear friend who was beloved by all who had the privilege to work with him.
“We send our deepest condolences to his family for this immeasurable loss.”