The deal was engineered by venture capitalists The Carlyle Group, which has invested in the deal in return for some equity in the enlarged group. Bigmouthmedia offers paid search as well as search engine optimisation and has been an acquisition target for some time.
Bigmouthmedia chief executive Steve Leach will sit on the Global Media board and will remain in charge of UK and US operations. He will also become chief strategy officer for both brands.
Leach said: "It's no secret we have had a number of approaches for a deal, but it wasn't just about the investment - we needed to find a partner with shared chemistry."
Bigmouthmedia will employ more than 100 people in its London, Edinburgh and New York offices, and at Global Media's Manchester office. Global Media has a further 100 staff in Munich, Milan, Moscow, Oslo, Paris, Seoul and Stockholm.
The company will have more than 300 clients, including Hilton, British Airways, AOL and Barclays.