Mendelsohn will join the existing partners in the business, Sid McGrath, Ben Bilboul and Dave Buonaguidi.
Mendelsohn left Grey last week, four years after joining from Bartle Bogle Hegarty.
Karmarama was launched in January 2000 by the former St Luke's duo of Buonaguidi and Naresh Ramchandani. It currently handles business for clients such as Amstel, Coleman's and Kickers.
For the full story, see this week's Campaign.