The Scot joined the Omnicom shop in January 2007 with a brief to integrate TBWA and Tequila's creative disciplines, and has since helped secure new accounts such as Sentosa Resorts World and Singapore Airlines.
Phil Brett, the agency's chairman, commented: "Graham has done exactly what he set out to achieve and he felt it was time to move on. He will be leaving us amicably in search of the next opportunity. He is a very ambitious person."
Kelly will remain with the agency until November, during which time he will continue to work with senior creatives Ng Pei Pei, Gary Steel and Hagan De Villiers.
He has enjoyed a colourful career at a number of agencies including BBH Japan, Saatchi & Saatchi Singapore and Ogilvy & Mather.
Kelly said: "I had three key objectives when I joined: improve our creative standards, make the agency more integrated and win some major new accounts. With all the creative awards we’ve won over the last 18 months, and having built a more integrated creative department - plus the SIA and Resorts World wins - I’ve met the targets I set myself."