This comes as part of a recruitment drive by the agency to broaden its resource and to staff recently won accounts.
Monahan will run the agency on a day-to-day basis, allowing Rob Smith, the agency's current managing partner, to focus on new business and on developing Farm's non-core businesses. Monahan will also take responsibility for the recently won Quorn and Thomas Cook accounts.
Karl Bates, a planning partner at Grey, has also joined the agency as a senior planner and David McLagan, previously a freelance creative, has been hired as a creative director for Uncle Terry, Farm's direct marketing arm.
Gregor Findlay and Kevin Colquhoun join from the Scottish agency Family as a senior creative team.
Smith said: "2005 looks to be the most exciting year we have had yet at Farm, and the team we now have in place is going to make that happen."
Owen Lee, one of the creative partners, added: "We've looked long and hard to find talented people who bring fresh thinking to Farm. All the latest members of our team have experience beyond just London."