Mattson will lead strategy on the O2 business and help the agency make use of data and content in the planning process.
He has been at UM London since 2006, joining as an account director. Mattson has worked with brands including Dairy Crest, Marie Curie, Royal Mail, AIG, Mizkan, General Mills, and FSC.
Before this he was a planning manager at PHD Media for 18 months, and has also worked at Burkitt DDB and Upward Brown Media.
Mattson will report to Ed Cox, the managing director at Forward Media, and joins the company this week.
Forward Media was set up in 2014 after Havas picked up the £200 million European Telefonica account from ZenithOptimedia without a pitch in October that year.
Ed Cox, the managing director at Forward Media, said: "Tony is a top strategic thinker with a proven track record, and exactly the sort of talent we need as we move from our start-up phase into year two as a business.
"He’s going to make a big difference to both our clients’ business and to how we develop and grow as agency."