Mr. Williams was celebrated for delivering nuanced performances as swaggering street toughs, charming family men and smooth-talking gangsters. He played bootlegger Chalky White on the Prohibition-era HBO series “Boardwalk Empire” and earned four Emmy nominations as a supporting actor, most recently for playing an abusive, alcoholic father in HBO’s “Lovecraft Country.”

Mr. Williams was working at his mother’s day-care center, his acting career floundering, when he was cast as Omar. He had seen echoes of his upbringing in the character, although when a prop person handed him a shotgun during his first day on the set, Mr. Williams “didn’t know which end was which,” showrunner and “Wire” creator David Simon told the New York Times in 2017. “Mike is a beautiful man, but a gangster he is not.”