She was previously a managing partner at BJL, formerly Barrington Johnson Lorains, the UK's largest independent regional agency, and was among a group of senior executives who bought the agency two years ago.
Cheethambell JWT has been without a managing director since Brian Watson left to pursue other interests two months ago, having spent almost 20 years working for JWT in Manchester and London.
David Bell, the Cheethambell JWT chief executive, said: "Jane has always been somebody we've been interested in hiring and, when the managing directorship became available, she was the only name in the frame. Not only is she very energetic and commercially astute, but she has the skillsets we need."
Clancy began her career 17 years ago as an account director at Saatchi & Saatchi before returning to her native North West to join the then JWT Manchester. Between spells at McCann-Erickson Manchester and Roose & Partners, she joined PZ Cussons where she worked with GGT to launch the Carex range in the UK.
Clancy said: "The agency combines the JWT branding with the entrepreneurial skills of Cheethambell. Our digital, direct and design operations are all going well for us."