Murphy, 35, a managing partner and head of account management, has been with the agency since its launch in 1995 and has been closely involved with its key events both pre- and post-merger.
He has worked on accounts including the founding clients Virgin and News International and, more recently, led the successful Lego pitch.
RKCR/Y&R's joint chief executive Jim Kelly said: "James represents the future of the agency but significantly he's also been involved in all our past successes. He brings youth and enthusiasm and a strong sense of identity for the agency."
Murphy, who has shared the managing partner title with Marcus Brown, Tony Harris and Chris MacDonald, was unwilling to go into detail about any changes he was planning. However, he said: "I'm determined to strengthen the agency's fast, flexible creative approach, which is built round a non-departmental and flat structure; we feel that it is unique among the top ten agencies."
"We've had a good year this year: we've had a number of wins, we've hired people and we haven't lost anything. But the constant challenge as an agency gets bigger is to hold on to its distinctive DNA. I would expect to see some restructuring in certain disciplines," he added.
The RKCR/Y&R founders will conclude their earnout period at the end of this year. However, Kelly said that it was too early to say what any of their respective plans were once this watershed was reached. "Conversations are just starting but they're at an early stage. There won't be any full-blown contractual negotiations until spring or early summer," he said.
Murphy started his career in advertising in the post room at Bates UK in 1991.
He joined RKCR/Y&R as an account director in 1995, later rising to become the agency's head of account management.