Larry was adopted from the Battersea Dogs and Cats Home in London 10 years ago, and has served under three different prime ministers: David Cameron, Theresa May and Boris Johnson.
The cat was adopted in the wake of a large black rat being spotted near the Cabinet Office during a live television broadcast.
Larry was the first cat to hold the "Chief Mouser" title since his predecessor, a feline named Humphrey, retired in 1997.
The cat has been spotted frequently at 10 Downing Street and in the nearby area, often making appearances during live news broadcasts.
Larry's anniversary comes just a few months after Palmerson, the British Foreign Office's Chief Mouser, retired in August 2020 after being employed to hunt for rodents since 2016.