Hallam will join Initiative in January 2018 and will report to chief executive Richard Morris, working closely with him on the strategic direction of the agency.
Morris joined the Interpublic media agency in September after 22 years at Dentsu Aegis Network. Upon joining Initiative he was tasked with transforming the agency's offering in the UK.
Hallam, who is also a managing partner at Mindshare, has been head of strategy for a year at the agency. Before that, he was head of digital strategy and joined Mindshare in 2013 as head of social strategy. He has also worked at creative agency VCCP, where he was social director.
At the end of 2015, David Grainger left Initiative as head of strategy to join M/SIX as chief strategy officer.
Morris said: "Our mission is to take the initiative in culture for our clients and at the heart of this is our talent within the agency. With Jed, I know I have a wealth of strategic experience and ability."
This year the agency has picked up a number of media account wins, including Unibet owner Kindred Group and Carlsberg's global business.