The agency, which was formed in June by merging AIS London and Arnold KLP, is moving from Havas Media Group into Havas Creative Group from January.
Louise Whitcombe, group managing director at Havas Helia, will run Field Day which, Havas said, will continue to operate as a separate brand with its own clients.
The move will "facilitate easier links with other creative businesses creating more opportunities both for employees and clients", according to Havas.
Barnsdale joined AIS 2010 as business director, and Stratford joined in 2003. Geoff Gower, executive creative director, left the agency three months ago.
Field Day's clients include National Express (customer engagement), Cineworld (above-the-line) and Reese’s and Triumph (experiential).
There is currently 45 staff at the shop. When the agency was formed, there were 80 employees.