Potts, a copywriter, and Jex, an art director, will report to the creative partners, Kate Stanners and Paul Silburn, and will focus primarily on the agency's Cadbury account.
Saatchis now handles the advertising for Cadbury's Creme Egg, Flake and Buttons brands after the company consolidated its business into the SSF Group in September 2008.
Potts and Jex will also work across a number of Saatchis' other clients, which include Visa, Olay and T-Mobile.
Their appointment brings to an end a five-year stint at Mother, during which time they produced the Pot Noodle campaign featuring Welsh miners, the pitch-winning Stella Artois campaign and an ad for Zoo magazine, which was awarded a gold for best viral at Creative Circle.
Silburn said: "Rob and Andy will prove to be a great asset. Personally, I've been wanting to work with them for several years as I've always been impressed by their fresh thinking, their versatility and attention to detail."