In his sophomore year (2003), Washington played in nine of Florida State’s 13 games including the Orange Bowl. He was the second leading rusher with 387 yards and a 5.2 yards per carry average despite missing four games after dislocating his right elbow in the first quarter of the season opener against North Carolina. Washington’s punt return for a touchdown against Wake Forest was the first by a Seminole since Peter Warrick’s 59-yard return against Virginia Tech in the 2000 Sugar Bowl. He rushed for a season-high 121 yards on 17 carries against NC State, the highest total by a Florida State running back during the season. He scored the game-winning touchdown in the second overtime against the North Carolina Wolfpack on a 12-yard run to clinch the Seminole’s 11th ACC Championship in 12 years as a league member. Washington rushed for 69 yards on 13 carries in Florida State’s victory over Virginia and 65 yards on 15 carries in the Seminoles’ victory over Florida.

Washington attended Andrew Jackson High School where he proved himself an exceptional all-around athlete, dominating in football as running back, cornerback, and wide receiver. In his senior year, he rushed for 2,437 yards and 28 touchdowns, was a threat on kick and punt returns, returning three punts and one kickoff for touch-downs, and defensively had 88 tackles (52 unassisted) and three interceptions. After this superlative season, in 2002, he was named “Mr. Florida” in football, and earned the Florida Times-Union Player of the Year honor. The Orlando Sentinel named him the No. 3 prospect overall in the state of Florida, and Alliance Sports named him the No. 7 prospect overall in the nation. He was named to the PrepStar Dream Team, rated by Rivals100.com as the No. 1 cornerback in the country and the No. 9 player overall, and rated by TheInsiders.com as the No. 7 cornerback in the country. He was named the Florida Kids’ No. 28 prospect from the state of Florida, named to Bill Buchalter’s Florida Super 26, was recognized on the Athlon Sports Top 100 High School Seniors, SuperPrep’s Top 100 nationally, and Max Emfinger’s Top 200.

Washington received an athletic scholarship to attend Florida State University, where he played for the Florida State Seminoles football team from 2002 to 2005. As a freshman in 2002, he changed positions from cornerback to running back. That year, he played in all 14 games and ranked fourth on the team with 273 yards rushing while catching six passes for 30 yards. He led the team with 273 total return yards and also in punt return average (11.5 yards per return) and kickoff return average (28.3 yards per return). He became the first true freshman at FSU to record a 100-yard rushing game since Travis Minor in 1997. He was named ACC Specialist of the Week twice for his performances against Clemson and Duke, he led the Seminoles in rushing in the Sugar Bowl vs. Georgia with 48 yards on 10 carries, he returned a kickoff 97 yards for a score against Clemson, he recovered a blocked punt in the endzone for a touchdown against Duke, and finished the year with 11 tackles on special teams with two forced fumbles and one fumble recovery.

During his junior year in 2004, Washington earned the Gator Bowl Most Valuable Player honors with 12 rushes for a career-high 195 yards in the Seminoles’ victory over West Virginia. His 16.3 yards per rush is a single-game Florida State record while his 195 yards is the second most by a Seminole player in a bowl game. He earned All-ACC second team honors and was named as the Seminoles’ offensive Most Valuable Player by the coaching staff at the team banquet. He played in 10 of 12 games while earning 10 starting assignments, leading the ACC in rushing yards per game with an average of 95.1. In addition, he finished second in the ACC with 112.5 total offensive yards per game, while leading the team with 951 total yards of offense and seven rushing touchdowns. He ranked second nationally in average yards per carry at 6.89. His season totals included 14 receptions for 98 yards and four kickoff returns for a total of 81 total yards. He scored a total of 42 points to rank second on the team behind only kicker Xavier Beitia.

In his senior year season in 2005, Washington played in 11 games and started in 10, missing two games because of an ankle injury he suffered in the Maryland game. Washington became the only player in the Bobby Bowden era to score touchdowns in five different ways—by run, reception, punt return, kick off return and fumble recovery. He ranked second on the team in rushing yards (430) with a 4.4 yards per carry average, and averaged 10.7 yards per catch. He was the 10th player in FSU history in career rushing with 2,041 yards. He had a season-high 179 all-purpose yards when he rushed for 87 yards and had 92 yards receiving in the Wake Forest game, including a career-long 61-yard touchdown reception. Washington returned six punts for 51 yards in the 2005 season and returned four kickoffs for 63 yards. He led the team in rushing in the Orange Bowl (against Penn State) with 30 yards on six carries, and added six catches for 24 yards in that game. Washington played in 43 games over his FSU career, and was named the Seminoles’ most dependable running back in spring practice in 2005 by the coaching staff.

Washington’s 2006 Bowman “Signs of the Future” card with the Jets caused a great deal of controversy during the third week of November 2006, in which it appears that he is making an obscene gesture. The card sold on eBay during that time for more than four times its book value. Washington insists his gesture is a popular hand gesture among his friends in his hometown.

On October 22 at home against Detroit, Leon ran for 129 yards for two touchdowns and led the Jets’ to a 31–24 win. In a game in Miami on Christmas night, Washington had 108 receiving yards including a 64-yard reception to set up the game-winning field goal in a 13–10 win. He helped the Jets clinch the fifth playoff spot in the AFC on New Year’s Eve 2006 with a touchdown run that helped to seal a win against the Oakland Raiders in the final game of the 2006 regular season. He finished the regular season with 650 yards rushing and four touchdowns on 151 attempts. He averaged 4.3 yards-per-carry. In an AFC East game, he returned a kick 92 yards for a touchdown against the New England Patriots in 2008. Washington gained 2,317 all-purpose yards in 2008, more than any other running back in the NFL.

In a Week 9 game against the Buffalo Bills on November 2, 2008, Washington made a play as a kick returner when he noticed a kickoff was headed out of bounds at the 8-yard line. When he saw the ball would stay in bounds, Washington straddled the sideline and then put his hand on the ball. The officials initially spotted the ball at the 8-yard line before Washington reminded them that, according to the rulebook, the ball is out of bounds when a player standing out of bounds touches it, meaning the correct call was Illegal procedure. This gave the Jets the ball at the 40-yard line, a 32-yard gain in field position.

Washington was selected as AFC Special teams Player of the Week for Week 11 of the 2008 season, the first such award in his career. His 92-yard touchdown was the fourth kick return touchdown of his career, surpassing Justin Miller for the club record. He led the league in all-purpose yards in 2008 with 1606 yards.

Washington was out for the 2009 season with a compound fracture to his fibula that he suffered in Week 7 in a 38–0 shutout win versus the Oakland Raiders. Prior to his season-ending injury, Washington had rushed for 331 yards on 72 carries, with a 4.6 yards/carry average. Washington had yet to score in 2009 prior to his injury. The Jets call their wildcat formation “Seminole” because Washington lined up at quarterback, and was a Florida State Seminole.

On April 15, 2010 (the NFL free agent deadline), Washington signed his tendered contract with the Jets for one year, worth $1.759 million.

On April 24, 2010 Washington was traded to the Seattle Seahawks for a fifth-round draft pick in the 2010 NFL Draft. He was assigned the number 33 on his new team; 29 went to first-round draft pick Earl Thomas. During the September 26, 2010 game against the San Diego Chargers, Washington set a Seahawks record with 2 kickoff return touchdowns. He returned the opening kickoff of the 2nd half for a team record 101yds, then in the 4th quarter he ran 99yds for his second kickoff TD of the game. On March 1, 2011 Washington signed a four-year deal worth $12.5 million to stay with the Seattle Seahawks. Due to the signing of wide receiver Percy Harvin, Leon Washington was released by the Seahawks on March 12, 2013.

On March 14, 2013, Washington signed with the New England Patriots. On September 1, 2013 Patriots released Leon Washington during final roster cuts. On September 7, the Patriots re-signed Washington. On November 23, 2013, he was released.

On November 26, 2013, Washington signed with the Tennessee Titans.

Washington re-signed with the Titans to another one-year deal on March 12, 2014.

On March 11, 2019, the Lions hired Washington as WCF minority coaching assistantship/offense and special teams coach.