Walker played for the Boston Celtics, Dallas Mavericks, Atlanta Hawks, Miami Heat, Minnesota Timberwolves, the BSN's Mets de Guaynabo and the NBA D-League's Idaho Stampede before retiring from basketball in 2012.

Walker attended Mount Carmel High School, in Chicago, where he played with future NFL quarterback Donovan McNabb and attained all-state status as a senior, earning a full scholarship to play for Rick Pitino at the University of Kentucky.

The following season, Pitino was reunited with Walker as the new coach of the Celtics and he made the All-Star team as a reserve while averaging 22.4 ppg and 10.2 rpg.

Just 10 days before the start of the 2003–04 season, Walker was traded with Delk to the Dallas Mavericks for Raef LaFrentz, Chris Mills, Jiří Welsch, and a 2004 first-round draft pick.

Despite appearing and starting in every game for Dallas that season, Walker was used in a relatively limited capacity on his new team due to the number of high-caliber players populating the Mavericks' roster.

On February 24, 2005, Walker was traded back to the Celtics in a deal that sent Gary Payton, Tom Gugliotta, Michael Stewart, and a first-round draft pick to the Hawks.

For his second stint with the Celtics, Walker briefly chose to wear the uniform number 88, as his trademark 8 jersey was being worn by rookie Al Jefferson.

On August 2, 2005, Walker was involved in a five-team, 13-player deal (the largest trade in NBA history) that sent him from the Celtics to the Miami Heat.

Walker also led the team in overall 3-point shots made for the season, and increased his 3-point, free throw and overall shooting percentages.

On October 24, 2007, Walker was traded to the Minnesota Timberwolves along with Michael Doleac, Wayne Simien, a first-round draft pick and cash considerations in exchange for Ricky Davis and Mark Blount.

[7] After appearing in just two preseason games and remaining inactive throughout the opening months of the regular season,[8] he was waived by the Grizzlies on December 19, 2008.

In February 2010, Walker signed with the Guaynabo Mets along with former Chicago Bulls player Marcus Fizer.

[16][17] On July 15, 2009, Walker was charged with three felony counts of writing bad checks related to gambling debts he had incurred at three Las Vegas casinos.

Walker was in South Lake Tahoe to play in the American Century Celebrity Golf Classic the following day.

[21] In July 2000, Walker and former University of Kentucky teammate Nazr Mohammed were robbed outside a South Side Chicago restaurant.