• by Steve McClellan  @mp_mcclellan, March 28, 2013

After a 21-year run with the company Thomas L. Harrison is stepping down as chairman of Omnicom Group’s Diversified Agency Services division, effective April 1, the New York-based holding company confirmed Thursday.

The company said that Harrison would remain an adviser to the DAS group and would take the title of chairman emeritus.

Omnicom Group CEO John Wren credited Harrison for bringing “entrepreneurial passion, strategic insight and counsel and excellence to his leadership role at DAS.”

DAS is a collection of over 200 specialized marketing services firms that focus on health care marketing, public relations, customer relationship management and other capabilities. CRM alone accounted for more than $5 billion in revenues for Omnicom in 2012, according to the company’s annual report.

All specialty communications -- including CRM and PR -- contributed about $7.5 billion to Omnicom’s coffers last year, or more than 52% of the company’s total revenue.

Harrison joined the outfit in 1992, when Omnicom acquired Harrison & Star Business Group, a health care marketing communications group that he co-founded. He was appointed president of DAS in 1997 and became chairman and CEO in 1998.

DAS began a phased leadership succession plan in late 2011, when company veteran Dale Adams took over the CEO reins while Harrison remained chairman. The two executives have worked together at the company for 15 years.

In addition to his DAS advisory duties, Harrison will continue to serve on a number of outside boards, including those of Montefiore Hospital, Morgan’s Hotel Group, Dipexium Pharmaceuticals, the New York Academy of Medicine and the New York Academy of Sciences.