Named a high-school All-American by Parade magazine and USA Today in 1995, Alexander earned the nickname “Mr. Touchdown” early in his career. He went on to star at the University of Alabama (1996–99), where he rushed for 291 yards in a single game as a freshman and became the school’s all-time leading rusher, gaining 3,565 yards in a career that also included 50 touchdowns. He was chosen by the Seattle Seahawks with the 19th overall pick in the 2000 NFL draft.
In 2001 Alexander began a streak of five consecutive 1,000-yard-plus seasons by gaining 1,318 yards—including a Seahawks record of 266 yards in one game, among the highest single-game totals in NFL history. The next year he recorded a franchise-record-breaking 16 touchdowns. Yet it took several seasons for him to step into the limelight nationally. In the process he became the first player in league history to score 15 or more touchdowns in five consecutive seasons. For the 2005–06 season he was named Associated Press Offensive Player of the Year and NFL Most Valuable Player after setting an NFL record for the most touchdowns scored in a season (28; a record that was broken the following year by LaDainian Tomlinson) while also leading the Seahawks to their first Super Bowl appearance in franchise history. In 2006 he negotiated an eight-year contract with the team for $62 million, the highest amount for a running back to date in NFL history. Alexander’s on-field production decreased significantly over the following two seasons, however, and injuries forced him to miss games for the first time in his professional career. After averaging 89.6 and 55.1 rushing yards per game in 2006 and 2007, respectively, he was released by the Seahawks in April 2008. Alexander signed with the Washington Redskins in October 2008. He appeared in only four games with the Redskins and was released the following month.