Freya, Chancellor George Osborne's cat, has been rescued by a charity worker after straying more than a mile from her Westminster home.
Homelessness outreach worker Kate Jones found Freya in Vauxhall, identifying her from a tag on her collar.
Ms Jones said Freya, who has since returned to Downing Street, was "a gorgeous cat and, just like the people we work with, deserved some help in getting home".
Freya has lived at No 11 since 2010.
Freya somehow ended up south of the river, travelling at least a mile and a half away before being discovered.
She spent the night with Ms Jones, who works with Thames Reach's London Street Rescue, before being collected by Downing Street officials.
Ms Jones said the cat was "exceptionally friendly" but clearly frightened.
"I have a lot of experience in helping homeless people off of the street and last year my organisation Thames Reach helped over 1,000 rough sleepers escape a life of rough sleeping," she said.
"I'm not so used to working with homeless cats but when we saw Freya we realised how distressed she was, we did the only right thing possible, and helped her get off the streets too."
Freya has reportedly gone missing before after disappearing from the family's Notting Hill home in 2009.
Cats have become a familiar feature of life in Downing Street over the years.
David Cameron welcomed Larry into No 10 in 2011 to try to solve a recurring rodent problem, although his mousing credentials have since been questioned.
Eyebrows were raised when Mr Osborne added a dog to his No 11 household in 2011. Lola, a young bichon frise, was said to be cat-friendly.