Willis played before his NBA career for the Jackson Community College and Michigan State University. In the 1984 NBA Draft he was selected by the Atlanta Hawks in eleventh place. He played nine full seasons and two additional games to another season with the Hawks, with whom he often moved into the NBA playoffs. In 1994 he moved to the Miami Heat. Later he came over the Golden State Warriors, Houston Rockets, Denver Nuggets and Toronto Raptors to the San Antonio Spurs, with whom he 2002/ 03 won the championship. In 2004 he went back to the Atlanta Hawks, for whom he was an active season. After he then played two years at any NBA club, Willis signed on 2 April 2007 a 10 -day contract with the Dallas Mavericks.
Willis is one of 15 players who could accumulate in their NBA career than 16,000 points and 11,000 rebounds. In 1992 he was nominated for the Eastern Conference All-Star team.
- 39 points of 11 March 1986 against the Denver Nuggets
- 16 transformed throws from the field (this succeeded twice)
- 31 attempted throws on the field on 13 April 1993 against the Cleveland Cavaliers
- Two transformed 3-point throws ( this succeeded him three times)
- Four attempted 3-point throws ( this succeeded twice)
- 12 free throws turned on February 14, 1992 against the Orlando Magic
- 16 attempted free throws on 31 March 1994 against the Sacramento Kings
- 16 offensive rebounds on 19 February 1992 against the Washington Wizards
- 20 defensive rebounds on 3 December 1991 against the Dallas Mavericks
- 33 rebounds on 19 February 1992 against the Washington Wizards
- Eight assists on 18 February 1994 against the Los Angeles Clippers
- Six blocks on January 7, 2001 against the Seattle SuperSonics
- 55 Minutes played on 13 April 1993 against the Cleveland Cavaliers