Mustarde will lead its planning department and join the London agency management team alongside the managing director, James Hayhurst, and the creative director, Nigel Roberts.
Hayhurst joined the agency from DDB in March, while Roberts arrived from CHI & Partners last year.
Mustarde, who has recently worked as a freelancer, was previously a planner at Abbott Mead Vickers BBDO and Bartle Bogle Hegarty, where he wrote its 2008 IPA Effectiveness Grand Prix paper for Johnnie Walker.
He most recently led the planning for the "escape the map" campaign, created by AMV for Mercedes-Benz.
At Leagas Delaney, Mustarde will drive an effectiveness agenda and contribute to its new-business effort.
He said: "Leagas Delaney has a hunger for great work and a new management team with the drive to deliver it."Follow @Darbs73