After three years at Maryland, Williams decided to leave for the NBA. The New Jersey Nets selected him third overall in the 1981 NBA draft, behind Olympic teammates Aguirre and Thomas. In his first season with the Nets, he averaged 15.5 points and led the team with 12.3 rebounds per game, helping New Jersey win 20 more games (a 44-38 win-loss record) than the previous year and earning 1982 Rookie of the Year honors. Williams established himself as a premier player at the power forward position over the next eight seasons with the Nets; in six of those he was ranked among the best three rebounders in the league, never averaging less than twelve rebounds per game. 1983–84 featured the Nets’ first playoff second-round appearance since the ABA-NBA merger in 1976, when they lost to the Milwaukee Bucks, and the Nets failed to subsequently get past the first round until 2002 when Jason Kidd led them to an unsuccessful NBA Finals date.
On June 24, 1989, the Nets traded Williams to the Portland Trail Blazers in exchange for Sam Bowie and a draft pick. In Portland, Williams would continue his solid play and take a complementary frontcourt role to established guard duo Clyde Drexler and Terry Porter. The Blazers’ post-season campaigns ended in the first round four consecutive seasons prior to 1990; contrastingly, Williams’ first three seasons with the Blazers were marked by three Western Conference Finals appearances and two NBA Finals. In 1990 the Blazers succumbed to the powerhouse Detroit Pistons in five games, while in 1992 they fell to the Chicago Bulls in six. Williams was regularly in the starting lineup for the first six of his seven seasons with the Blazers. He remains the franchise leader in field goal percentage (55.0%)
In the twilight of his career, after the 1995–96 season, Williams moved back to the Atlantic Division, signing with the New York Knicks, where he played in a much more limited capacity, behind the frontcourt duo of Patrick Ewing and Charles Oakley. He spent two years with the Knicks, but was forced to miss 41 games during the 1997–98 season due to knee surgery (the first time in his career he missed more than 12 games in a season). Williams announced his retirement on January 27, 1999, holding career averages of 12.8 points and ten rebounds per game and a field goal average of 54.9 percent. During the course of his 17-year NBA career, Williams racked up more than 16,000 points and 13,000 rebounds — one of only seven NBA players to ever reach both marks.
|1981–1989||New Jersey Nets|
|1989–1996||Portland Trail Blazers|
|1996–1998||New York Knicks|
|Career highlights and awards|
|Career NBA statistics|
|Points||16,784 (12.8 ppg)|
|Rebounds||13,017 (10.0 rpg)|
|Assists||1,646 (1.3 apg)|