As deputy ECD, Cartmell reports to Mahoney, who joined the WPP shop as chief creative officer in February, and will support the day-to-day running of the creative department.
Cartmell, who joined from Karmarama last year as a creative partner, will also join the agency’s broader management team to help build and drive a vision for the agency.
The agency’s senior team also comprises: Charlie Rudd, chief executive; Kevin Chesters, chief strategy officer and Clare Donald, chief production officer – all of whom have been appointed within the last year.
Cartmell will continue to have responsibility for key clients across the business, having worked across Pizza Hut Delivery, BP and Heineken UK.
Mahoney said: "Charlie, Kev, Clare and I have exciting plans for O&M London and we are in unanimous agreement that Sam is the kind of person to help us deliver them.
"He is a talented creative with diverse experience and he gets what we’re trying to do here at O&M. He also happens to be a lovely bloke to have around."