The pair, who joined Karmarama three years ago from Fallon as creative directors, will report to the founder and chief creative officer, Dave Buonaguidi.
Since joining Karmarama, Walker and De Souza have worked on a number of campaigns for Costa, Kerrygold, Carnival Cruise Lines and Plusnet. Walker has also directed TV work, including Plusnet’s "days of fibre" ad.
They will now be tasked with leading the output across Karmarama’s 38-strong creative department and attempting to raise the standards of the work it produces.
Walker and De Souza spent three years as a senior creative team at Fallon, where they created award-winning work for Orange, including a number of Gold Spots and the "dance" ad.
Before that, they were at Mother, where De Souza won a D&AD black Pencil for his Britart work.
Buonaguidi said: "Since joining the agency, Sam and Joe have been responsible for creating some of our most high-profile work. Their recent work for clients such as Costa and Carnival rank among the highlights on the agency’s showreel."