Duffy joined the City PR agency from Interbrand in October 2011 in a newly created role to run the firm's business day-to-day.
It is understood that Duffy left Tulchan at the beginning of May in what agency founder Andrew Grant called a 'mutual decision'.
Grant explained: 'Neil realised that Tulchan was not the right company for him and we acknowledged that he had not been the right hire for us, so we amicably agreed to part company.'
Grant said he remained convinced the agency would benefit from a managing partner and that Tulchan was actively recruiting for a replacement. It is thought the agency will recruit externally.
Duffy had responsibility for managing Tulchan's existing businesses in London and Singapore as well as supporting the development of sister Tulchan companies, Vox and Investor Perceptions.
At brand consultancy Interbrand, Duffy was a director, Europe, among other positions during his six years with the firm. He has also held senior management and commercial roles at Omnicom advertising firms BBDO and DDB, and specialist comms network DAS.
At the time of Duffy's hire, Grant said: 'We see enormous opportunities for continued growth, while maintaining our strong culture and enjoyment of what we do. However, if we are to fulfil that potential we need an experienced, professional manager to complement the entrepreneurial spirit that has brought Tulchan this far.'
Earlier this year, Tulchan replaced its existing non-executive directors Alison Canning and Howard Dodd.
Hilary Tuppen, an existing partner at Tulchan, and private equity specialist Hamish Mackenzie both joined the agency's board in January.