The agency said it will recruit a new chief strategy officer who will work with Giles Hedger, the chief executive who joined from FCB Inferno in May, and chief creative officer Justin Tindall.
Bell had owned a joint share in a 30% stake sold to the London ad agency at the beginning of last year, following her promotion to chief strategy officer.
The deal, which was based on a valuation of the ad agency of £25m, meant equity shares were given to then chief executive Tom Bazeley, Tindall, managing director Tom Firth, chairman Tim Duffy, and Bell.
Bazeley left in August 2016 and in May was replaced by Hedger, who took an equity stake in the business.
She leaves M&C after nearly four years in senior strategy roles, including head of strategy from 2014 until her promotion to chief strategy officer in January 2016.
Bell had previously spent five years at DraftFCB London (and then FCB Inferno after it acquired Inferno in 2013), first as brand planning director and from 2011 as global strategy lead for Beiersdorf.
Bell did not specify details about her new venture but said: "I am working on an exciting new venture of my own that I believe will have great traction in the market place and it needs dedication.
"I have really enjoyed my time at M&C Saatchi and I wish my fellow directors the best of luck and continued success."