Her exit follows a turbulent period for the auditor, with its UK office increasingly under pressure from its rival Billetts. This culminated in the loss of the lucrative COI Communications auditing brief.
Hainsworth was the second managing director at Media Audits in as many years. She joined the company in June 2003 as a member of the senior management team and was promoted to managing di-rector last July. She replaced Mike Larkin, who moved to an international role in charge of new business.
Hainsworth is a former managing director of the now-defunct Mediahead, which was formerly the media department of CDP. She is the second senior figure to leave the auditor in the past couple of months - Tony Emment, a former TV buyer at Starcom Motive, also left the company earlier this year.
Julian Spooner, Media Audits' chief executive, would not comment on Hains-worth's departure. Separately, he scotched rumours that shareholders have put his company up for sale.
He said: "We're not up for sale but we've had a number of unsolicited approaches. We continue to grow the business until we find a buyer that is right for the company, our people and our clients."