Williams, a former star at Dunbar who has coached previously in Prince George's County, was introduced to the team Thursday, athletic director Jeff Palumbo said in an interview.
Williams, 45, said he learned several weeks ago that the school where he coached this past season - Jericho Christian Academy in Landover - was closing.
"They shut down the whole high school. It was a surprise to everyone," Williams said. "We had gone 24-6 this season and we came within nine points of Montrose Christian."
Williams played on powerful Georgetown teams during the 1980s. As a freshman, he scored 19 points to help Patrick Ewing and the Hoyas beat Houston in the 1984 national championship game. At Dunbar, Williams played on the undefeated 1982-83 team considered one of the city's best ever. It featured other future NBA players Tyrone "Muggsy" Bogues and the late Reggie Lewis.
"I was very surprised how much the kids knew about him [Williams]," Palumbo said. "They talked about Dunbar and said, 'Mr. Palumbo, that's Baltimore basketball history.' "
Said Williams: "With this online stuff, they Google your name and find out everything about you."
Outgoing coach Josh Pratt said he learned in February that he was being laid off from his position as a physical education and health teacher at the school.
He said he wanted to coach and teach at the same place and has accepted a job as girls basketball coach at St. Vincent Pallotti.
Towson Catholic was 17-13 last season, finishing third in the Maryland Interscholastic Athletic Association and fourth in the Baltimore Catholic League.
Williams played 11 NBA seasons, ending his career in 1996-97 with the New Jersey Nets. He owned a restaurant in Bowie after his playing career ended.
As a coach, he said: "I'm kind of a combination of all the coaches I've played for. Defense and rebounding is our anchor."
Pratt, who was an assistant at Pallotti, Archbishop Spalding and St. Mary's before taking over at Towson Catholic for longtime coach Mike Daniel in 2005, went 98-40 (.710) in four seasons with the Owls. He guided them to two MIAA A Conference titles and one Baltimore Catholic League crown. Pratt's teams were ranked in the top 10 in The Baltimore Sun's final poll in each of his first three seasons, including a No. 1 ranking in 2007.