Kerrigan attended Purdue University, where he played for the Purdue Boilermakers football team from 2007 to 2010. As a freshman in 2007, Kerrigan appeared in 12 games recording 18 tackles and a sack. As a sophomore in 2008 he started 11 of 12 games and was an honorable mention All-Big Ten selection after recording 56 tackles, seven sacks and an interception. As a junior in 2009 he was a second team All-American by Rivals.com and a first team All-Big Ten selection. He finished the season with 66 tackles and 12 sacks. Kerrigan was also awarded Purdue’s “Pit Bull Award” in 2009, which was given to the player that exemplified and displayed tenacity and tough play.
Kerrigan was a semifinalist for the Rotary Lombardi Award during his senior season.
Kerrigan was selected by the Washington Redskins with the sixteenth overall pick in the 2011 NFL Draft. On July 29, 2011, the Redskins officially signed Kerrigan to a four-year, $8.72 million contract with a $4.842 million signing bonus. Kerrigan was converted from a defensive end to an outside linebacker. He was made the starting left outside linebacker. During his NFL debut on September 11, 2011, Kerrigan tipped up and intercepted an errant pass from New York Giants quarterback Eli Manning, subsequently scoring on a 9-yard scamper into the end zone. On September 29, the NFL named him Defensive Rookie of the Month for September. His rookie season was marked by dominant play. Some speculated Kerrigan would win Defensive Rookie of the Year. At the end of the 2011 season, Kerrigan had started in all 16 games and recorded 63 combined tackles, 7.5 sacks, four forced fumbles, one interception, and one touchdown. On January 16, 2012, Kerrigan along with fellow rookie Roy Helu, were named to the Pro Football Weekly/Pro Football Writers of America all-rookie team.