Thomas will work alongside creative director Damian Kirby and joins from Rapp, where he was a creative director.
Prior to joining Rapp in May 2008 he was creative director at New Zealand agency Clemenger BBDO Direct.
Masterman was formerly managing director of VA London, the digital agency he set up nine years ago.
The two other hires include Aimee Welstead as business director, who joins from Copan International, and Laura Kinghorn as finance director, who joins from PR firm Munro & Forster.
Mark Smith, director of Table 19, said: "We've spent a lot of time finding the right people to help us continue as one of the UK's fastest growing agencies."
Table 19 was founded in February 2008 by Smith, a former managing director of MRI, and former Sky customer marketing head Matt Broekhuizen.
Clients include Sky, Carphone Warehouse, The Geek Squad, Fulham FC and Ladbrokes.