Sadiq Khan has pledged £15,000 to Streatham Hill Theatre in support of plans to revive the venue.
The money will be put towards the crowdfunding campaign launched in January by Friends of Streatham Hill Theatre.
The campaign aims to repurpose the 2,800 capacity theatre as an arts and culture venue, and intends to raise £36,000 by May 25.
It has achieved £11,295, independently of the mayor’s pledge.
Theatres Trust director Jon Morgan said: “Streatham Hill Theatre is a theatre at risk that has great potential to reopen for theatrical use.
“The lavish interior is still in an extraordinary condition and it is a great shame that the public do not get to see it.”
He added: “There are not many theatres of this scale that could be restored and this funding from the Mayor of London will help the friends group establish a route to getting this building active once again.”
The theatre, which has historically been used as a bingo hall, is currently on the Theatres Trust theatres at risk register.
The grade-II listed building, which closed in 2017, is recognised by Lambeth council as an “asset of community value” which protects it from development.
Lambeth cabinet member for equalities and culture Sonia Winifred said: “This contribution from the Mayor of London Sadiq Khan is great news for Streatham, and great news for Lambeth.
“Streatham Hill Theatre is an important local asset that we want to see working for our community, as part of our commitment to preserving our heritage and culture in our diverse borough for future generations.”