The pairing will work alongside six other creative group heads and report directly to the chairman and creative director, Trevor Beattie.
McClelland replaces Ben Priest, who quit the agency in January to be the creative director at Rainey Kelly Campbell Roalfe/Y&R.
McClelland, 35, has worked as a copywriter at BBDO and Leo Burnett in New Zealand, followed by BBDO in Australia. For the past five years, he has been at DDB, where he has worked across the majority of the agency's accounts, including Marmite, The Guardian, Nikon, Oxfam and London Transport.
His recent work includes the latest Volkswagen "30 years in the making" ad to launch the new Golf, and the latest brand campaign for Harvey Nichols - "HN on Earth".
Throughout his career, he has won a raft of awards, adding to them this week at the Campaign Press Awards with a gold and silver for The Guardian, a silver for Heals, and a commendation for Nikon.
Beattie said: "I promised Brian we'd hire him the finest copywriting talent around. Patrick is simply the best.
Brian and Patrick will form a formidable creative team.
"I've made no secret of the fact that I'm planning to build the most potent creative department in the business Patrick's hiring is another huge step towards that goal."