Slater, 28, takes up his new role with immediate effect. He takes over from Sarah Gold, who will now concentrate on her joint managing director's role.
He has been with the agency for two years, working on the Heineken and Carphone Warehouse accounts.
Latterly, he has assisted Gold with the agency's new- business efforts.
Gold was promoted to the joint MD role, in partnership with Neil Goodlad, in June.
Johnny Hornby, the managing partner at CHI, explained: "Last year, Sarah's efforts saw CHI reach the top of the new-business performance league.
"After her promotion, we needed to hand day-to-day responsibility of new business to someone good and I'd be surprised if there was a better young account man in London than Ben. He did a great job of supporting Sarah on a number of our pitches last year."
Slater, who was a Campaign Face to Watch in January, started his advertising career in the postroom at Abbott Mead Vickers BBDO. He was promoted to account management and worked on the agency's BT and Guinness accounts.
He also worked in the new-business department. CHI hired him in April 2002.
Slater added: "CHI has a brilliant new-business record and I very much hope that in my new role I can take the agency from strength to strength."
The agency's new-business wins last year included British Gas, BT and Talk Talk.