Williams was selected by the Cleveland Cavaliers in the 1985 NBA draft with the 21st pick of the second round (45th overall). However, due to the trial, Williams spent the 1985–86 season playing for the United States Basketball League. Able to play for the Cavaliers the next year, Williams was named to the NBA all-rookie team for the 1986-87 season, along with teammates Ron Harper and Brad Daugherty. Perhaps Williams' finest season occurred in 1989, when he averaged 16.8 points, 8.1 rebounds and 2.04 blocked shots per game while mostly serving as the team's sixth man. Following the 1989-90 season, he re-signed with the Cavaliers to a 7-year, $26.5 million contract, making him one of the five highest paid players in the NBA in the early 1990s. At the time, this was an unprecedented salary for a sixth man like Williams.