Cecil now goes by the name of Stephenson following her marriage two years ago.
Her departure will see existing MD Alix Robson, who is supported by deputy MD Tania Fawcett, take charge of the fashion and luxury goods PR specialist.
In the past year, Cecil - who plans to explore 'new opportunities', including writing a book - has spent just one day a week in the office.
Cecil, who set up Aurelia PR in 1990 at the age of 23, sold her agency to advertising group Abbott Mead Vickers in 1997 for £4.25m (PRWeek, 8 August 1997).
Next month is the end of a five-year earn-out period from the 1997 deal, and Robson said that Cecil's departure had been 'on the agenda' for 18 months.