After graduating from Chicago's Neal F. Simeon High School, Simmons played three years of college basketball for DePaul University. During his three seasons at DePaul, Simmons averaged 13.6 points per game, 7.5 rebounds per game and 2.2 assists per game.
Simmons was selected in the second round (41st pick overall) of the 2001 NBA draft by the Seattle SuperSonics. Simmons's draft rights were traded to the Washington Wizards for Predrag Drobnjak on June 27, 2001.
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. He was released by the Pistons on September 24 and, less than two weeks later, signed with the Wizards again on October 10. Later, Simmons signed a free-agent contract with the Los Angeles Clippers on September 26, 2003.
The 6'8", 235 lb 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. He was rewarded for his stellar play with the NBA Most Improved Player Award.
He signed a free-agent contract with the Milwaukee Bucks on August 8, 2005, shortly after the new collective bargaining agreement was finalized. Since his breakout year with the Clippers, Simmons has struggled to reach the same level of play. In his first year with the Bucks, his points, rebounds and shooting percentages all declined. Then in 2006, an ankle injury forced him to miss the entire 2006–07 season.
Bobby 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. While his season was not as productive as he personally would have liked, he still managed to finish 5th in the league in 3 point shooting percentage.
In September 2010, Simmons signed with the San Antonio Spurs. He was waived by the Spurs in November 2010.
In March 2011, Simmons signed with the Reno Bighorns of the NBA D-League.
On February 27, 2012, the Los Angeles Clippers signed Bobby Simmons to a 10-day contract. On March 9, 2012, Bobby Simmons signed a second 10-day contract with the Clippers. He signed with them again on March 24, 2012.
In December 2012, Simmons said that he was still interested in playing professional basketball.
Simmons and Lavelle Sykes founded a Chicago clothing store called Succezz. Bobby Simmons also owns a nightclub in Chicago called Society.