The 10-year veteran, who has spent his entire career with the Washington franchise, bid farewell to the team on Instagram late Sunday night.
The 32-year-old's playing time was greatly reduced in 2020.
Though he played in all 16 games, he started in just one. Kerrigan was also on the field for 57% of Washington's defensive plays in 2019, and just 38% this past season. His 397 total defensive snaps were a career-low in 2020, and the pass-rusher's 94 special-teams snaps were a career-high.
He recorded 17 total tackles and 5.5 sacks in 2020, both career-lows.
Kerrigan reportedly requested a trade in November.
The four-time Pro Bowler owns the franchise record for sacks with 95.5.