Mabox will provide both attractions, which are part of the Imperial War Museum, with full-service marketing and PR support.
The focus will be on marketing the museums' events and visitor promotions during the year in order to attract new UK and overseas visitors, repeat visitors, more families and special interest groups.
Upcoming events that will be given marketing support include a September Harvest Fair at the Churchill Museum and a shipbuilding exhibition at HMS Belfast.
The Royal Navy cruiser's role in D-Day, where she was one of the first ships to fire on German positions at 5:30am on June 6 1944, will also be highlighted.
Denise Wild, account director at Mabox, said: "The attractions market is highly competitive and we have a challenge to raise their profiles in the London and South East market through the implementation of creative marketing campaigns."