She steps down as a part-time senior consultant at the agency, having been central to a number of big wins, including Loyalty Management UK, which operates the Nectar scheme.
Elliot also helped secure the agency's recent win of Voca – formerly the Bankers' Automated Clearing Service – having worked with it as UK chief executive at Manning Selvage & Lee.
She will now focus on her own company, corporate PR firm Cathcart Consulting. Its clients include Cancer Research UK and the British Medical Association.
'The move suited both of us and it was a good time to part company,' said College Hill chairman Alex Sandberg. 'We part on good terms and Jackie was an asset to the firm during her time here.'
Industry veteran Elliot was formerly global PR chief at watchmaker Rolex. Named in PRWeek's '100 most influential figures in the industry' last year, she acted as PRCA chairman from 1996-98 and chief executive of MS&L, which she joined in 1980, until 2001. She led MS&L's global health practice and was a steering group member on the UN Economic and Social Council.
l College Hill has been appointed by US litigation services firm FTI Consulting to handle corporate comms for its expansion into Europe. FTI last year hired ex-Football Association CEO Mark Palios as senior MD. The brief is said to be worth around £100,000.