Forde resigned from his role as Channel 4's new-media marketing manager last week and joins Rocket XL in April as its managing director.
At Channel 4, he handled social media campaigns for Skins and Lost, as well as the marketing for the broadcaster's websites.
Craig Howe, the founder and chief executive of Rocket XL, said: "We needed someone in the UK with the same level of insight and experience we have in the US market and Steve has seen what works well first-hand."
The London agency will operate as its European lead office, working with Cossette's UK group agencies - Miles Calcraft Briginshaw Duffy, Dare, Elvis, Identica, Brando and Band & Brown - and as a specialist full-service agency.
Cossette acquired a majority stake in Rocket XL last May.