Richardson was born in Chicago, Illinois, to Lee and Emma Richardson.He arrived at DePaul University after leading Whitney Young High School to the state AA boys Basketball title in 1998. He averaged 17.9 points per game, and 10.2 rebounds per game in his two seasons at DePaul. He became the only player in school history to have 1,000+ points, 500+ rebounds, and 100+ three-point field goals. As a freshman, he was elected both the Conference USA Player of the Year, and Freshman of the Year. Richardson declared for the NBA Draft after his sophomore year with DePaul in 2000
Richardson was drafted by the Los Angeles Clippers with the 18th pick of the 2000 NBA Draft. He was selected after fellow Clippers Darius Miles and Keyon Dooling. Richardson would star in a documentary with Miles entitled The Youngest Guns which chronicled their first three seasons in the NBA with the Clippers. Richardson spent four seasons with the Clippers before signing with the Phoenix Suns as a free agent.
The 2004–05 season was a big one for not only Richardson, but the Suns as well. He set a new Suns single-season record for three-point field goals, eclipsing the previous record of 199 set by Dan Majerle. He finished the season with a league-leading 631 three-point attempts, and 226 three-point field goals, co-leading the league with Kyle Korver. Richardson also set a Suns franchise record with nine threes against the New Orleans Hornets on December 29, 2004. Richardson would also go on to win the NBA All-Star Three-Point Shootout that same season. The Suns finished the regular season with a league-best 62 wins and 20 losses. He made his playoff debut with the Suns in 2005 who would eventually lose to the San Antonio Spurs in the Western Conference Finals.
Richardson was traded from the Suns, along with 2005 draft pick Nate Robinson, to the New York Knicks in exchange for Kurt Thomas and Dijon Thompson in the offseason. His first three seasons in New York were largely hampered by nagging injuries—the most serious being a chronic back condition—which limited him to 55, 49 and 65 games played respectively. His injury situation finally stabilized during the 2008–2009 season, when he remained healthy enough to appear in all but seven games. This does not include two additional DNP-CDs (Did Not Play – Coach's Decision) that he received; one on February 28, 2009 against the Miami Heat and a second on March 10, 2009 against the Milwaukee Bucks.
On the 2009 draft day, Richardson was traded to the Memphis Grizzlies in exchange for Darko Miličić. His stint at the Grizzlies only last three weeks before he was traded again to the team that drafted him, the Los Angeles Clippers, in exchange for Zach Randolph. His second stint with the Clippers only lasted for three days. On July 20, 2009, he was traded to the Minnesota Timberwolves for Sebastian Telfair, Mark Madsen, and Craig Smith. After less than a month at Minnesota, Richardson was traded for the fourth time in the 2009 off-season, this time to the Miami Heat for Mark Blount.
In 2010, he signed with the Orlando Magic. He remained with the team until October 2012, when he was waived. On April 16, 2013, Richardson signed with the New York Knicks for the remainder of the season, joining that team for a second time
- June 28, 2000: Drafted 18th overall by the Los Angeles Clippers in the 2000 NBA Draft.
- July 29, 2004: Signed as a free agent by the Phoenix Suns.
- June 28, 2005: Traded by the Phoenix Suns along with draft rights of 21st pick Nate Robinson to the New York Knicks in exchange for Kurt Thomas and draft rights of 54th pick Dijon Thompson.
- June 25, 2009: Traded by the New York Knicks along with cash considerations to the Memphis Grizzlies in exchange for Darko Miličic
- July 17, 2009: Traded by the Memphis Grizzlies to the Los Angeles Clippers in exchange for Zach Randolph.
- July 20, 2009: Traded by the Los Angeles Clippers to the Minnesota Timberwolves in exchange for Sebastian Telfair, Mark Madsen, and Craig Smith.
- August 13, 2009: Traded by the Minnesota Timberwolves to the Miami Heat in exchange for Mark Blount.
- July 13, 2010: Signed with the Orlando Magic.
- October 27, 2012: Waived by Magic.
- April 16, 2013: Signed by the New York Knicks.