Ahead of World Mental Health Day on 10 October, Cllr Helen Dennis, Southwark’s deputy cabinet member for mental health, swam 3 km to raise money for the Dragon Cafe.
Swimming 3 km in the outdoors, she has raised £470 to date and is hoping to push the total over £500.
Founded by the late Sarah Wheeler in 2012, the cafe offers creative activities to help people find mental wellbeing.
"I'm really delighted to lend my support in this way to a wonderfully creative organisation, which is enabling Southwark residents to directly access and get involved in so many different activities to support their wellbeing," said Cllr Dennis, who is a Labour councillor in Chaucer ward.
"When I visited the Dragon Cafe, I saw people engaged in art, creative writing and singing, and heard about mindfulness, sport and gardening groups. Above all, I was struck by what a friendly and welcoming place it was.
"If anyone is struggling with their mental health at the moment, I would really encourage them to seek out support, contact their GP or visit the Wellbeing Hub. This kind of project shows that you're not alone."
On the swim, Cllr Dennis added: "The swim didn't go completely to plan – I had hoped to swim a 5 km earlier in September but illness and injury set me back.
"Given the circumstances, I'm very proud of the achievement and I hope that it will encourage others to make use of Southwark's free swim & gym offer, or even sign up for the Mayor's 'Duckathon' later this year!"