Slater, who is working out his contract at CHI, will move to New York in August to take up his new role. The details of his new responsibilities have yet to be finalised, but he is expected primarily to work in new business, with some account responsibilities.
Slater will work with Gwyn Jones, the former chief executive of BBH London, who departed to run the agency's US office in 2004.
Slater joined CHI as an account director in 2002, working on Heineken and Carphone Warehouse.
He was promoted to new-business director in August 2004, retaining responsibility for the Talk Talk account and more recently adding Britvic and Full Tilt Poker to his client list. Johnny Hornby, a partner at CHI, said: "We've all loved having Ben in the agency - he has been a big part of our success and making it a fun place to work. As I see it, we are lending him to Bartle Bogle Hegarty until we've got an office in New York."
Jones said: "We are very excited about Ben joining BBH here in New York. His experience driving organic growth in a young agency will make him a fantastic addition to our team."
Slater, who was a Campaign Face to Watch in January 2004, started his advertising career at Abbott Mead Vickers BBDO in 1998. He was promoted to account management, working on Dulux Paints and BT before being promoted to account director on Guinness. He also worked in new business.