Born in Toronto, Ontario, Jamal Mayers began his hock career by playing Junior B level hockey with the Thornhill Rattlers. Mayers was then chosen 89th overall in the 1993 NHL Entry Draft by the St. Louis Blues after his freshman year at Western Michigan University. He returned for three more years of college and reached double figures in goals each time. In the 1996-97 season, Mayers played six games for St. Louis but spent the bulk of his time developing with the American Hockey League'sWorcester Ice Cats.
Mayers began the 1998-99 season in the minors again, but was recalled at mid-season by St. Louis. A solid defensive player in the last 34 games of the season, Mayers then showed no signs of nervousness during 11 post-season games that season. In the 1999-00 season, Mayers scored seven goals and played on the fourth line while helping the Blues finish at the top of the NHL's regular season standings. The next year he registered 21 points and was used in a variety of situations. Mayers was particularly solid in the playoffs when St. Louis advanced to the Western Conference final against the Colorado Avalanche. The Blues went on to lose that series, but it was a career accomplishment for Mayers and his teammates. Mayers began the 2001-02 season as a valuable enforcer and power foward on one of the league's elite franchises and was one of the team's more durable players, having only missed 13 games in his last three seasons. However, a serious knee injury would cut Mayers' season short. The Blues had another strong season, but Mayers was limited to only 15 regular season games.
Mayers had played his entire ten-year career with the Blues before he was traded to the Maple Leafs in exchange for a third round pick in the 2008 NHL Entry Draft on June 19, 2008, one day before the draft itself. During the 2008-09 season, Mayers has filled a valuable leadership role with the young Maple Leafs, as his experience is invaluable to instructing the emerging players of the franchise. Mayers has typically occupied the right wing position on the Maple Leafs fourth line, which has allowed him to utilize his efficient forechecking skills and physical play in a reduced playing capacity. However, in a December 1, 2008 game against the Los Angeles Kings, Mayers suffered a broken right hand after colliding with the opponent's goalpost in a scramble around the Kings' net.
On the international stage, Mayers represented Canada at the 2000, 2007 and 2008 World Hockey Championships.