The digital agency will be rebranded as Momentum and its 50 staff will be moved into the Momentum offices.
The deal, which took more than a year to finalise, was completed on Tuesday (8 April), and the company started trading as a single entity on the same day.
Nick and Jon Hamm, the brothers who founded Greenroom, will both take the role of head of digital/executive creative director. The Momentum senior management, including the worldwide chief executive, Chris Weil, will keep their existing titles.
The deal will see Momentum increase its digital capability and presence in the entertainment sector while giving Greenroom a chance to take its offering global and expand out of the digital space.
Paul Biggins, the chief executive of Momentum London, said: "We didn't have the depth and breadth of digital skill, but this deal allows us to plug that gap."
Nick Hamm added: "There are no conflict issues, which means the merged companies will be much greater than the sum of their parts."