Allen will retain an ongoing part-time consultancy role to a portfolio of agencies, whilst TRO’s group board will remain unchanged, with Michael Wyrley-Birch continuing to run the EMEA region.
Allen was appointed managing director in 1996 having spent his career both on the client-side and in advertising and consultancy roles.
During his time at the agency, he led the sale of TRO to global marketing group Omnicom in 2008, bringing together teams in Manchester, Norwich and London.
Allen also served six years on the board of Eventia where he twice held the role of chairman.
Allen said: "For just over 18 years I have had the absolute pleasure of enjoying a leading position within TRO, but I now feel that the time is right for me to move into a different role so that I can free myself up for a number of other things I want to squeeze into my life.
"I shall be undertaking some small scale consultancy work within the industry, which will include work for TRO, as well as two or three other businesses."
Keith O’Loughlin, chairman and chief executive officer, TRO, added: "Rob is a well-known personality throughout our business and the UK events community.
"He is known for his passion for the events industry, individual commitment, and to have a genuine care for the work he does and the people he works with."
In October, TRO reported a 33% rise in turnover to £32m, while profits fell slightly to £1.7m.