Being measured at just 6'0", Harrison was believed to be too short to play as a linebacker in the NFL, which was the primary reason he went undrafted out of Kent State.
Following the 2002 draft, Harrison was signed to the Pittsburgh Steelers practice squad, where he stayed for two seasons. During that time, Harrison was released and resigned multiple times by the Steelers.
Following the 2003 season, Harrison was picked up by the Baltimore Ravens, only to be later released. It was at this time that Harrison considered retiring from football to pursue a career as a truck driver. Harrison was only 26 years old.
Then came the Steelers again.
Harrison signed with the Steelers for the fourth time. It was at this point Harrison's career took off.
During the course of the 2004 season, Harrison received playing time on special teams and defensive end, as well as at linebacker.
Not until the 2007 season would Harrison be named a starter at linebacker. During this season, Harrison terrorized the football field registering 8.5 sacks, seven forced fumbles and 93 tackles on the year. In his first season as a starter, Harrison was voted to his first Pro-Bowl.
The years following, his performance has been exceptional and is widely considered one of the top linebackers in the NFL. His consistently high level of play has made him a perennial Pro Bowler, being voted to five consecutive All-Star Games.