Under Waley-Cohen's management the 403-seat venue, which is currently hosting Stephen Thompson's political satire Whipping it Up, starring Richard Wilson, will be known as the Ambassadors.
Waley-Cohen also runs the Victoria Palace and St. Martin's Theatre, home to the world's longest-running play, The Mousetrap. Agatha Christie's whodunit opened at the Ambassadors in 1952 before moving to its neighboring current home in 1974.
In a statement, Rosemary Squire, ATG's joint chief executive and the current president of the Society of London Theatres, said, "We are delighted that this wonderful West End venue is in Sir Stephen's hands - one of the most reputable and successful men in the theatre industry today. We hadn't planned to sell the venue, but the offer made good commercial sense at a time that coincides with our overseas expansion and the launch of our New York office."
Waley-Cohen is a former president of SOLT and previously managed the Savoy and Vaudeville theatres.
Responding to speculation, Waley-Cohen said that he has no immediate plans to transfer The Mousetrap back to the Ambassadors.