Mahorn was dubbed by Piston announcer George Blaha the "Baddest Bad Boy of them all." Mahorn gained a reputation for physical play, which he used to compensate for his relatively limited leaping ability. He served as a team leader of the Detroit Bad Boys teams of the late 1980s, winning his only NBA Championship in 1989 along with captain Isiah Thomas, Joe Dumars, and Dennis Rodman among others.
In 1989, Mahorn won his only NBA championship with the Pistons. Though the Bad Boys went on to repeat in 1990, Mahorn was picked up in the 1989 NBA expansion draft only days after hoisting the '89 trophy, as teams were only able to protect 8 of their players from being "drafted." After he was selected by the new Minnesota Timberwolves, Pistons general managerJack McCloskey tried in vain to trade to get him back. In ESPN's 30 for 30 feature film about the Detroit teams in this era, Mahorn shed a tear when talking about being dealt away from the Pistons. Despite being out of Detroit, Mahorn never played for Minnesota, being traded instead to the Philadelphia 76ers, where he teamed with superstar Charles Barkley (despite previous rivalries with him) to form the top-rebounding duo of "Thump N' Bump." After two seasons, Mahorn moved to the Italian Serie A league for the 1991–92 season.
Mahorn later played for the New Jersey Nets for four seasons, before returning to the Pistons in 1996–97 under coach Doug Collins. He retired after the 1999 season, after a second stint with the 76ers. Mahorn then served as a color commentator for Pistons radio broadcasts, and as an assistant coach under former teammate Bill Laimbeer with the WNBA's Detroit Shock. Laimbeer and Mahorn led the Shock to multiple WNBA titles.
On July 22, 2008, at a Sparks-Shock game, Mahorn attempted to break up a brawl. When attempting to restrain Lisa Leslie, he put his left hand out and Leslie fell to the ground. Mahorn was suspended for two games.
On June 15, 2009 he became the head coach of the Shock, a position he held until the franchise moved to Tulsa, Oklahoma after the season. Shortly afterwards, Mahorn continued his work with Pistons radio, doing color commentary alongside Mark Champion.