O'Rourke said he took the decision so he could pursue other interests, including his role as governor at the Arts Institute in Bournemouth. He relinquished his role as managing director at Terence O'Rourke in 2003 to continue as chairman.
The firm recently completed a management buyout process to retain its independence, which led to founding directors Ron Gorman and Chris Clarke leaving the board to carry out different roles in the practice.
Managing director Tim Hancock said: "We hoped that Terry would also follow this strategy and we are saddened that he has not been able to do so."
The buyout process has resulted in 78 of the employees becoming shareholders and allowed the business to expand, Hancock added.