The move sees Keith Nichol, one of Flourish's three founders, move from creative director to executive creative director.
The Bristol-based agency describes Nichol's new role as a more client-facing role identifying creative opportunities early in the strategic planning stage.
MRM is currently evaluating how to fill the gap left by the departure of Forsyth, who was one of its three creative directors.
Forsyth was at MRM London, where he worked on Intel, Microsoft and Sky, for just over two years and has over 17 years' experience working in numerous London agencies.
Prior to his time at MRM he was head of copy at Draft, where he won awards for Oxfam's 'I'm in' campaign.
Flourish's clients include Scope, Samiritans, Coca-cola, Camelot and Wickes and recent wins include Dreams and Friends of the Earth.