The move sees the two former creative directors from Evans Hunt Scott reunited, and comes as JWT looks to respond to client demands for integrated ideas across the media mix.
The decision comes despite JWT's acquisition of the below-the-line agency Black Cat last year.
The two will be charged with working beyond the boundaries of traditional advertising, to offer clients integrated creative solutions. Muir and Howard will take the title of integrated creative directors and both report directly to the agency's executive creative director, Jaspar Shelbourne.
Muir previously worked with Howard at EHS from 1995 to 1997. After leaving EHS, Muir joined FCB San Francisco as the agency's integrated creative director alongside JWT's now chief executive, Simon Bolton. Muir left the agency in 2000, and after a spell of freelancing joined JWT a year ago.
Howard founded Zinc, EHS's interactive agency, in 1996. He left earlier this year.
Bolton said: "Many agencies struggle with the idea of creative integration.
This team construct demonstrates JWT's desire to create ideas that work across many customer contact points."