He replaces Phil Mahoney, who will leave PHD after 14 years to take up an international investment role at WPP’s Maxus.
Mackenzie will join in December and will sit on the PHD board, reporting to Danny Barnes, PHD’s head of investment.
He has worked at Vizeum since 2003, holding the titles of broadcast manager and associate director of TV, before becoming the agency’s head of broadcast. Prior to Vizeum, he worked at ZenithOptimedia and BBJ.
Barnes said: "James has a wealth of experience across a range of categories which fit our client base perfectly. He’s an exciting buying talent and we’re delighted to be bringing him on."