Johnson inherited a new feline colleague at the Foreign and Commonwealth Office as well as his new role in the Cabinet -- and his cheeky streak appears to have rubbed off on the cat.
Palmerston took his Downing Street ambitions further by sneaking in to Theresa May's new residence while no one was looking.
10 Downing Street chief mouser Larry is believed to be "away" for a few days and power hungry Palmerston took the opportunity to dash into the home, media reports said.
He then sat outside the building, looking furious and even appeared to have a stand-off with a police officer.
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Amid the post-referendum upheaval within the government, Palmerston and his No. 10 rival Larry have been at each others'
throats recently.
One violent clash outside the Foreign Office is thought to have resulted in a paw injury to Larry which required veterinary treatment.
No. 10's cat was put on bed rest for a few days as he licked his wounds.
Although recent comings and goings in Downing Street have been linked with the feline skirmishes, Larry does have a history of troubled relations with his neighbours.
He often came to blows with the cat from No. 11, Freya, who was then exiled to the countryside.
Police were previously forced to step in to break up a cat fight between Larry and Freya, former Moggy of the Treasury.
Larry arrived at No. 10 in 2011, having been picked out by staff from Battersea Dogs and Cats Home.