The pair will work mainly on the agency's B&Q account, but will also cover a number of other clients and work on new business.
Smith and Butler have worked on a freelance basis since 2008.
Previously, they ran the creative department at Grey, before effectively being replaced by Andy Amadeo.
The pair have also worked at Lowe, where they created "Stunt City".
Smith and Butler have picked up a number of awards, including Cannes gold Lions for "Stunt City" and "through the bottle" for Smirnoff.
Brian Fraser, the executive creative director at McCann Erickson London, said: "Our hiring of Geoff and Simon further cements our dedication and creative ambition for the B&Q account, and we are extremely excited about the work to come."