Clelland will work with Hudson Sandler vice-chairman Andrew Nicolls, which whom he worked at Penrose, the agency founded by Nicolls.
Nicolls said: "We were an extremely effective combination at Penrose and I look forward to achieving the same success again, delivering a world class service to our current and future clients."
Clelland, a former Reuters journalist, has worked in both agency and in-house roles at companies including Fitch Ratings, Weber Shandwick and RBC Dexia.
He said: "I am very pleased to be joining Andrew at Hudson Sandler, a strategic and collaborative consultancy at an exciting stage in its development. I look forward to building on its success and that of our clients with the growth of the financial services practice."