Leigh is set to depart the agency next Friday. His resignation follows months of discussions between him and Langdon. Leigh's co-managing director, Simon Toaldo, will now take sole day-to-day control of the agency.
Langdon said: "We are really sorry to lose someone of Adam's great talent. Simon leads a line-up that is a match for the best agencies in town."
Leigh joined the agency in 2000 as its deputy managing director. He was promoted to managing director in 2002, sharing the job with Toaldo after Euro merged with Partners BDDH last summer.
Langdon offered Leigh a number of roles within the company, including the deputy chairmanship.
Leigh said: "Now is the perfect time for me to move on."