Julie Brown joins the Oxfordshire-based agency's London office in June from the Waste and Resources Action Programme (Wrap). Her title is yet to be decided.
Wrap will be one of her clients at the agency.
'Julie's appointment will allow the agency to expand into the environmental sector, targeting bodies or businesses that have a environmental concerns or impact,' said 3 Monkeys senior director Katie Evans.
This business growth is likely to come from the London office, she added.
Brown's role at Wrap has now been split into two: a head of PR responsible for a media relations team and management of an agency roster; and a head of comms overseeing direct marketing and campaigns.
Procurement communications manager Judith Moore has moved into the latter post. Wrap is recruiting for a replacement head of PR.
Brown, who will report to founder Angie Moxham at 3 Monkeys, said: 'My role at Wrap has been split in recognition of the increasing role that comms and PR have to play in the organisation.' She previously worked at Icas PR.
3 Monkeys has also appointed Laurence Rosen – Office Angels founder and former MD of recruitment agency Alfred Marks – as a non-executive director. He is working part-time as a consultant on client services and business development.