The Lambeth-based museum was founded in the former church of St Mary's in 1977 as the world first museum dedicated to the history of gardening, and it is still the only one of its kind in the UK.
Last year alone, over 47,000 visitors enjoyed its diverse events and a unique collection of around 6,000 objects that span 400 years of gardening in Britain.
The grant from City Bridge Trust will be used to fund the construction of a learning studio to help engage the local community with activities focused on horticulture and biodiversity. Horticultural clubs, cooking for children and other outreach programmes are being planned.
The Garden Museum's head of operations, Benjamin O'Connor, has also announced this week he is leaving the museum on 27 August for a job as director of The City Centre for the City of London Corporation and New London Architecture.