Mullin played shooting guard and small forward in the NBA from 1985 to 2001. After playing at St. John's University, during which he won Big East Men's Basketball Player of the Year three times and was a member of the 1984 U.S.A. Men's Olympic Basketball team, Mullin was chosen as the seventh pick by the Golden State Warriors in the first round of the 1985 NBA Draft.

He played with the Warriors from the 1985-86 until the 1996–97 seasons. Thereafter, Mullin played with the Indiana Pacers from 1997 until the 1999–2000 season. He retired after the 2000–01 season, playing for his original team, the Warriors. In his 16 year career, Mullin was named an All Star five times, named to the All-NBA First Team in 1992, and was an All-NBA Second Team twice.

In 2011, Mullin was inducted into the Naismith Basketball Hall of Fame and the National Collegiate Basketball Hall of Fame. A year later in 2012, the Warriors retired Mullin's number 17. Since retiring, he has been an analyst for ESPN.