The group was formed this week by the merger of Los Angeles-based creative and sports marketing agency Troika and New York and London-based PR and experiential agency Mission (not to be confused with UK-listed Mission Marketing Group).
Forsyth’s role is global, but he will be based out of London, working alongside the UK chief executive Kevin Dundas.
Dundas, a former chief executive of Saatchi & Saatchi UK who headed the launch of Droga5’s London office in 2013, joined Mission around a year ago.
Forsyth, who left Adam & Eve/DDB last year, told Campaign he had considered joining a start-up after his year-long non-compete clause lapsed in May, but worried that "start-ups can’t start up fast enough to satisfy clients’ need for scale".
Troika/Mission will be headed by president Nicola Stephenson and chief executive Robert Machinist, and brings together clients such as Netflix, Victoria’s Secret and InterContinental Hotels.
Forsyth will provide strategic planning to clients and help shape the agency to meet modern client needs and "making sure we attract the right new clients as we grow the business both here in the UK and at a global level".
"I think clients are now looking for a full-service agency offering," Forsyth said. "The merger of these two agencies was far and away the most exciting option."
Explaining how he was introduced to the agencies, he said: "I’ve known Nicola since she set up Mission 15 years ago and a while back I was a non-executive to Mission. We’ve worked on projects together when I was at Naked and Adam & Eve."
Mission’s UK client roster is fairly retail-focused, and its past work includes establishing Amazon’s first Black Friday in the UK and launching upmarket clothing brand J Crew in London.
Forsyth added: "The formation of Troika/Mission Group brings together a ‘super group' of talent in the culture capitals of New York, London and LA to create a new agile, 24-hour communications agency with strong focus on cultural insight, influencers, consumer experience, digital innovation and online advertising."