The deal is the first of its kind and will include delivering documents such as council tax letters and polling cards to 1.4 million London addresses.
Royal Mail said councils will save money by centrally printing and then posting letters and documents to residents in the capital.
Terry Brewer, of the London Borough of Harrow, who chairs the London Postal Services Board, said: "The participating London boroughs are delighted with the results of the tender process, which has seen Royal Mail being awarded the contract.
"We look forward to working with Royal Mail to continue to provide a quality service and achieve further savings and efficiencies."
Royal Mail, which has been facing competition in the capital from rival firm TNT, said it was a significant contract, adding it was in discussions with a number of other London local authorities with a view to signing a second wave.