Starting in January next year, the creative team will take up the positions of head of copy and head of art respectively, with responsibility for all of the agency's clients. Procter & Gamble is one of Grey's biggest clients.
Smith and Butler's most famous recent work was on Sure's "Stunt City", but they have also worked on Tesco Cherokee and Reebok.
During their careers, they have picked up golds at Cannes, the BTAA Awards and Campaign Poster Awards. They have also won silver awards at Campaign Press and D&AD.
They both started their advertising careers in the late 80s, with Butler joining Lowe Howard-Spink and Smith starting at Delaney Fletcher Slaymaker Delaney. They formed a team at Lowe in 1995, working mainly on the agency's Unilever brands.
Separately, Grey has appointed Alex White as its UK group chief financial officer. His remit will extend to all of Grey's UK companies, including Joshua, the brand design company Geometry, MDS Global Consulting and the PR company GCI.
He started his career at Butler Dennis & Garland in the mid-80s before holding posts at Young & Rubicam across Europe. He was also involved with the acquisition of Rainey Kelly Campbell Roalfe.
Tamara Ingram, the Grey UK group chief executive, said: "Alex has expertise in the financial management systems required to underpin world-class performance."