Stephens, 36, has been the managing director of Rapier for three years having previously worked as the managing director for the TBWA agencies Tequila Payne Stracey and TBWA Direct.
Referring to the resignation of the Government's health secretary, Alan Milburn, last week for family reasons, Stephens insisted that he was not "doing a Milburn and downshifting" but "moving outside of the direct marketing agency arena to broaden my skills and experience".
Rapier has no immediate plan to find a replacement for Stephens, who will stay with the agency until December. The only imminent change is the promotion of the business director, Catherine Ince, to the position of head of account management.
Stephens declined to give details of his future plans but is likely to launch his own venture. "It was a difficult decision because it's a fantastic job and a fantastic agency. But after one start-up, two mergers and three years at Rapier, I felt that it was time to broaden the horizons."
The agency, which specialises in work for services clients, has grown on the back of wins such as the AA and HSBC during Stephens' time as the managing director. Rapier also launched a consultancy division as it moved upstream. It is independently owned - the chief executive, Jonathan Stead, is the majority shareholder.
Stead, one of the original founders of Rapier, hired Stephens following the departure of Simon Toaldo to Partners BDDH. He said: "Ben has done a great job for us over the past three years in a tough business climate. I can understand, having spent seven years as an managing director, that he now wants to look outside our industry for new opportunities."
Rapier's senior line-up includes the creative director, John Townshend, and the planning director, Alison Meredith. The agency reported 2002 billings of £64 million and a staff of 87.