The £26m media planning account has been picked up by MEC following a competitive pitch.
It picked up the business from Manning Gottlieb OMD and Carat, who worked on the RAF and Navy accounts respectively.
The combined creative business was awarded to Engine in 2013.
“We’ll be working closely with both services and Number 10 [Downing Street] to ensure our campaigns attract the very best talent in the country,” said Stuart Bowden and Jason Dormieux, MEC chief executives, in a joint statement.
They said that winning the business represented a ‘real coup’ for the agency.