Denney replaces Alan Young who becomes chief creative officer and owner.
He will work closely with Young and Julian Vizard, creative partner and owner, and will work across the agency’s portfolio of accounts with the aim of leading and running his own as the agency grows.
Denney left MullenLowe in March as one of a string of high-profile names to exit the agency over the past year. His creative partner Dave Henderson, who was chief creative officer, left in September 2016. He subsequently joined Atomic.
Young said: "Richard’s tremendous track record and phenomenal energy adds real firepower to the department. His instinctively collaborative nature makes him the ideal match culturally."
Denney has also previously worked at DDB London, Rainey Kelly Campbell Roalfe/Y&R and Saatchi & Saatchi.
His awards include a silver and three bronze Lions at Cannes 2016 for the Persil campaign "Free the kids".