After graduating from Chicago's Neal F. Simeon High School, Simmons played three years of college basketball for DePaul University.

Simmons was traded by the Wizards on September 11, 2002, shortly before the start of the 2002–03 NBA season, to the Detroit Pistons with Hubert Davis and Richard Hamilton for Jerry Stackhouse, Brian Cardinal, and Ratko Varda.

The 6 ft 8 in (203 cm), 235 lb (107 kg) small forward had a breakout year during the 2004–05 NBA season with the Los Angeles Clippers in which he averaged a career-high 16.4 points per game.

Simmons signed a free-agent contract with the Milwaukee Bucks on August 8, 2005, shortly after the new collective bargaining agreement was finalized.

[4] Simmons was traded along with Yi Jianlian to the New Jersey Nets for Richard Jefferson on June 26, 2008, just a few hours before that year's NBA draft.

In September 2010, Simmons signed with the San Antonio Spurs; however, the team waived him in November after he went scoreless in 2 games played.