"He was a very serious impact player on this organization," said Peter Georgescu, chairman-CEO at Y&R Inc., the parent company for the Y&R ad agency and several other units.
Executives say there have been no discussions about how to fill Mr. Mosser's shoes.
Part of the problem stems from Mr. Mosser's vaguely defined role at Y&R, which he had joined 11 months ago after a 25-year career at its public relations unit, Burson-Marsteller.
Y&R Inc. named Mr. Mosser exec VP last January, tapping his expertise to handle corporate-branding issues for clients. More recently, he had been given account responsibility for Philip Morris Cos., a longtime Burson client whose subsidiaries are among Y&R's clients.
Just nine days before his death, Mr. Mosser was promoted to general manager of Y&R Inc., a previously vacant post now designed to help coordinate the resources of Y&R's various agency units.
But most important, Mr. Mosser was viewed as a "wise counsel" to Mr. Georgescu, helping to properly position the company and its broader communications role to clients and prospects, especially as Chairman Alex Kroll prepares to retire Jan. 1.
As one top official half-jokingly noted a week before Mr. Mosser's death, "Tom's job is to tell Peter what to do."
"He was a good consigliere," said Scott Flood, a former director of business development at Y&R, and now general manager at Altschiller & Co., New York. "He made clients feel very comfortable they were getting real top-flight advice and counsel from him."
Joe Mandese contributed to this story.