Hallmark (pictured), joined Manifest London in 2013 as an associate director, helped launch Manifest New York in 2015 and aided the development of the agency's content division, O.
Before joining the London office, he was head of digital and social at PR firm Kaizo.
Hallmark's new role will see him take responsibility for the London agency, while founder Alex Myers takes a role at Manifest Group, the parent company set up when Myers took full ownership of the firm last year.
Myers said: "Neil joined Manifest when we had one office and a team of 11. Since then, the agency has gone global, the team has almost tripled in size, we've built a content division and developed several new proprietary technologies."
Manifest's roster of clients includes games console brand PlayStation, independent Scottish brewer Innis & Gunn, and pub chain All Bar One.
Late last year, Manifest announced it had opened a third office, in Stockholm, amid plans to take the business global and set up further operations in South Africa and Australia by 2020.
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