Larry, a four-year-old tabby cat, arrived at his new home at 10 Downing Street, tasked with warding off a pack of rats seen scuttling close to the British leader's official residence.
The former stray, adopted from London's Battersea Dogs and Cats Home, is the latest pet to be appointed Chief Mouser to the Cabinet Office, an unofficial pest control post.
Television cameras had captured several sightings of rats around Downing Street -- a common problem in London, where the trash on sidewalks provides a constant supply of food.
"I'm delighted to welcome Larry to his new home," Cameron said in a statement. "I'm sure he will be a great addition to Downing Street and will charm our many visitors."
Cameron's office said the pets home believed Larry would be well suited to his new role because "before coming to Battersea, he was a stray, so he is used to fending for himself on the streets."
The last holder of Larry's office, a male cat called Humphrey, retired in 1997 and died in 2006.
Ex-Treasury chief Alistair Darling -- who lived next door to then-Prime Minister Gordon Brown at No. 11 Downing Street -- also had a pet cat, Sybil, who was renowned for her prowess in catching mice.
Cameron's spokesman Steve Field said Larry had taken a relaxed approach so far to his duties. "He's spent most of the time asleep, he seems quite relaxed and easy going," he told reporters.
Field confirmed Cameron had met with the cat and picked him up, but said it wasn't yet clear whether he would sleep in the family's quarters or elsewhere in the building.
Downing Street staff -- not taxpayers -- are picking up the costs of feeding Larry, Field said.