Brown Betty partnered with free newspaper Metro to run the competition, which gave it editorial coverage on the front of and inside the paper.
Readers were given a 'secret password', and Metro explained that if readers approached one of the four actors dressed as Lara Croft herself and told her the password, they would win a free a free iPad mini. Twelve were given out during the morning.
The newspaper also carried a review of the game, the latest in a long-running series featuring the action heroine Lara Croft.
Brown Betty co-founder Lynn Daniel said: "We know how important it is to delight our clients with our ideas, so we're thrilled at the success of this idea and the willingness of Londoners to join in with the stunt. When ideas are condensed down to their simplest, most creative form and executed well, that's when the magic starts to happen."
The agency has since sent photography from the stunt to national newspapers in the hope of further coverage.
Brown Betty was founded last year, and in addition to Lara Croft GO maker Square Enix, counts 2K Games, the BBC and Warner Bros among its clients.