Sian Berry said she would "break the grip" of big developers on London's housing market.
Zac Goldsmith has promised to build houses at the rate of 50,000 a year by 2020, while Labour candidate Sadiq Khan has pledged 80,000 new homes a year.
Launching her manifesto, Ms Berry also said she would support tenants with a London Renters Union.
The Green Camden councillor has laid out plans to halt the demolition of London estates, and ensure half the 200,000 homes would be built "affordably", by councils, smaller developers, communities and housing associations.
Ms Berry also promised:
Ms Berry said "fairness" was at the heart of the Green's policies, which meant "building more homes with co-operative and smaller companies... funding experts to help communities defend their estates and helping private tenants with rent controls and a renters' union so they can organise and stand together."
Sadiq Khan also emphasised housing affordability and fairness in his manifesto, which set a target of half of new homes built across London to be affordable, with Londoners getting "first dibs" on properties.
Conservative candidate Zac Goldsmith has pledged to ensure "a significant proportion" of all new homes are for rent, and says he will give Londoners the first chance to buy new homes in the capital.
A recent poll indicated housing was the most important issue for Londoners in the upcoming mayoral elections.
The Green Party mayoral candidate for London has vowed to bring in rent controls and build 200,000 new homes.