Martell Webster was born in Edmonds, Washington, in 1986. His mother, Cora McGuirk, disappeared in 1990, when he was four years old. It is suspected that she was a victim of Gary Ridgway, known as the "Green River Killer", a serial killer who who murdered dozens of women and girls in Washington during the 1980s and 1990s.
Webster was selected by the Blazers with the sixth pick in the 2005 NBA Draft after the Blazers' traded down their third pick to the Utah Jazz just hours before the draft. He was assigned to the Fort Worth Flyers of the NBA Development League by the Blazers in January 2006, and in doing so became highest drafted player (6th overall) to be assigned to the D-League until Hasheem Thabeet. He later returned to the Portland Trail Blazers in February 2006. He scored a season-high 26 points in a January 5, 2008 win over the Utah Jazz, with 24 of them scored in the third quarter. He is one of the last ever high schoollottery picks to be chosen in an NBA Draft due to new draft eligibility rules introduced in 2006. In October 2008, Webster signed a four-year, $20 million contract extension.
On February 20, 2009, it was announced by Trail Blazers athletic trainer Jay Jensen that Webster would likely miss the rest of the 2008–09 NBA season with a left foot injury, having only played 5 minutes during the season.
On January 23, 2010, he scored a season-high 28 points in a win against the Detroit Pistons. Webster was traded on June 24, 2010 to the Minnesota Timberwolves in exchange for Ryan Gomes and the rights to draft pick Luke Babbitt.
Webster underwent back surgery in October 2010 and missed nearly half the 2010-11 NBA season, leading Timberwolves General Manager David Kahn to charge the Trail Blazers with failure to adequately disclose a known injury. Another back surgery followed in September 2011, limiting Webster to just 47 games for the Wolves in the 2011-12 season. In the spring of 2013 Kahn's complaint was reportedly settled by the Blazers for $1.5 million just before the matter was brought to a formal hearing before the NBA. It was said to be among the largest cash settlements in such a case. Both teams were sworn to secrecy about the exact terms of the deal, according to basketball journalist Henry Abbott of ESPN.
On July 13, 2012, Webster was waived by the Timberwolves. He signed with the Washington Wizards on August 29, 2012 on a one-year, $1.6 million contract.
On March 16, 2013, Webster scored a career-high 34 points in a win against the Phoenix Suns, also tying another career-high with seven three-pointers.
Webster re-signed with the Wizards on July 10, 2013.