Dillion attended Franklin High School in Seattle, Washington, and was a two-sport standout starring in football and baseball. In football, he was an All-State selection and the All-Metro Player of the Year. He was also an excellent baseball outfielder, and garnered All-Metro honors, and was drafted by the San Diego Padres in the 1993Major League Baseballdraft.
At the University of Washington, Dillon set the team all-time single-season records for rushing yards (1,555 yards) and touchdowns scored (22) in 1996. In the first quarter against San Jose State University, Dillon rushed for 222 yards and caught an 83 yard touchdown pass, setting NCAA records for both rushing yards and all-purpose yards (305) in one quarter. Dillon did not re-enter the game as the Washington Huskies were comfortably ahead 36-0 by the end of the first quarter.
Dillon was originally drafted by the Cincinnati Bengals in the second round of the 1997NFL Draft. For six seasons, Dillon was one of the few bright spots on an otherwise mediocre Bengals team. From 1997 to 2002 he rushed for over 1000 yards each year, and made the Pro Bowl 3 times (1999–2001). He also set an NFL record for most yards rushed in one game (279 yards) against the Denver Broncos (the record was broken by Jamal Lewis, which was then broken by Adrian Peterson).
In 2003, Dillon only rushed for 541 yards due to injury, which, along with the emergence of Rudi Johnson, precipitated the trade of Corey Dillon to another team. Originally, he appeared to be traded to the Oakland Raiders, but the Raiders were unwilling to sacrifice a second round draft choice for the aging running back. The New England Patriots, on the other hand, willingly traded a second round pick to give Dillon a second chance on a team with a proven track record. Dillon left the Bengals as the team's all time leading rusher with 8,016 yards, easily surpassing the previous record of 6,447 yards set by James Brooks.
In the 2004 season, Dillon proved himself to be a mature and dedicated team player, putting to rest the negative reputation he may have acquired before arriving in New England. Moreover, Dillon had his best year with 1,635 rushing yards and 12 touchdowns, both career highs, and set a franchise record in the process. The year culminated in a third Super Bowl win for New England, in no small part because of the new running game built around Dillon. Dillon was the top rusher of Super Bowl XXXIX with 75 rushing yards and a touchdown, while also catching 3 passes for 31 yards, giving him 106 total yards. He was also a major factor in the Patriots win over the heavlily favored Indianapolis Colts in New England's first playoff game, rushing for 144 yards and catching 5 passes for 17 yards. Overall, Dillon rushed for a total of 292 yards, caught 9 passes for 53 yards, and scored 2 touchdowns in New England's 3 postseason games.
In 2005, Dillon was once again plagued with injury problems and was not able to duplicate his high stats from 2004. He remained a major contributor for the team, rushing for 733 yards and 12 touchdowns in 12 games. Dillon was also used more frequently during this season as a pass receiver, with 22 receptions for 181 yards and a touchdown, which was more receiving yards then he had gained in his past 2 seasons combined. At the end of the 2005 season, Dillon had amassed 10,429 career rushing yards, ranking him #17 on the NFL's list of all time leading rushers.