McClelland will work alongside Morris on projects across all Lowe's clients. These include Tesco, Stella Artois and Nestle.
However, McClelland will not take on any responsibility for the creative department, which Morris will continue to run single-handedly.
McClelland has been at TBWA for 15 months, as a creative group head on the PlayStation account. He was brought in to partner Brian Campbell and work alongside six other creative heads.
Before that, he spent five years at DDB London, where he worked for clients including The Guardian, Marmite and London Transport.
He was responsible for the Volkswagen "30 years in the making" commercial to launch the new Golf and also for the Harvey Nichols "HN on Earth" campaign. He picked up a silver Lion for the Harvey Nichols work at Cannes this year.
Previously, McClelland has worked at BBDO and Leo Burnett in New Zealand, followed by BBDO in Australia.
Morris said: "Patrick is one of the best writers in London. He has a lot to bring to a department that has a lot to offer."
McClelland added: "Danny Brooke-Taylor and Tony McTear (TBWA joint executive creative directors) are doing a fantastic job but this is a great career move that gets me higher up the ladder.
"Working with Ed is a great opportunity. He has such energy and enthusiasm."