The agency has been without a planning director since the departure of Sarah Ryder, one of the agency's four partners, in February this year.
Kelly receives equal equity to Leith London's remaining partners, Jeremy Pyne and John Messum.
Kelly, who started this week, had been the acting planning director and senior planner on Nescafe and Mastercard at McCann, where he had worked for two years. He was hired by Pascale Reed, McCann's planning chief, who left the agency in May.
Kelly joined McCann from TBWA/GGT Direct, where he was a senior planner on its NatWest business. Before that he was at RPM3. He started in the industry at KWS in 1991, staying for seven years while the agency merged with FCA, and then into Publicis.
Kelly will have a significant role in the agency's Carling business, but will also work across a number of the agency's other accounts.
His appointment is an important one for Leith London, as it has lost two of its founding partners since its launch in September 2000. Although Ryder joined the agency in May last year from Campaign Palace, she stayed only nine months and was the second of the four partners to leave after Paul Silburn, its joint creative director, moved to TWBA/London in November last year.
John Rowley, the chief executive of Leith, said: "We've spent a long time over this appointment. What was certainly reinforced to me is that the team should work really well together. Richard was the best fit, and the chemistry thing for clients is important."
Kelly said: "Leith is one of those generic brands in the smaller creative hotshop end of the business. It is generically dynamic and creatively committed. To be involved in that, and with the clients it has got, is a great opportunity."
Despite its rocky start, Leith London has stabilised to win the Clear Blue pregnancy kit business from Unipath and Goodfella's Pizzas, which it was awarded out of Partners BDDH recently on the strength of its original pitch idea. It has also been handed the Coors Brewers accounts for Carling and Reef, which moved from Roose in August.