William's practice saw off entries from Levitt Bernstein, RHWL, John McAslan and Burrell Foley Fisher to win the £24 million scheme in the heart of the historic cathedral city.
The Art Deco facade will be replaced by a multi-level glazed foyer which Williams describes as 'a crystal ribbon sitting in contrast to the [neighbouring] medieval and Georgian architecture'.
The winning designs also include plans to add a second, 200-seat theatre and expand the main 1,200-seat auditorium.
Rebuilt back-stage facilities, a remodelled flytower and new foyers and bars are among the other proposals.
Williams said: 'Naturally, I am delighted that I have won this prestigious competition and I am looking forward to developing our concepts with the city council and their partners to produce a world-class theatre for Canterbury.'
Work on site is expected to start in January 2009 and the 'transformed' Marlowe is due to open in autumn 2010.