The agency won the business from MediaCom without a pitch.
Generation Media will start work with Tomy on 1 July. Its first brief will be to support the launch of Tomy's Harry Potter range of toys, with a national cinema campaign to coincide with Harry Potter And The Deathly Hallows: Part 2.
MediaCom has worked with Tomy for nearly 15 years, and Dean Weller, the managing director of Generation Media, was part of the team that pitched for the business at MediaCom's predecessor, Media Business, in the 90s.
Weller said: "This is a major win for us, and gives us the opportunity to work on some great established brands as well as exciting new ones, including Teacup Piggies."
Weller said the win can make Generation Media the largest buying point for toys and games in the UK and Ireland.