He was drafted by D.C. United with the fourth overall pick in the 2002 MLS SuperDraft, but played only 28 minutes for DC in his first year as a pro.

The Chicago Fire acquired Mapp for Dema Kovalenko before the 2003 season, and he has been an important player in the squad since, helping them to the 2003 US Open Cup.

[7] On May 29, 2013, Mapp was awarded the 2013 George Gross Memorial Trophy for his play in the Canadian Championship, leading Montreal to victory over Vancouver Whitecaps FC in the final.

[11] Mapp has played for various youth United States national teams, including the 2001 Under-17 World Championship in Trinidad and Tobago.

Mapp also came on as a sub January 20, 2007, against Denmark, running 70 yards with the ball before setting up Jonathan Bornstein's game-winning goal.