Mitchem will join Nitro's former global executive creative director Paul Shearer and ex-managing director Johan Fourie in Arnold UK's senior management line-up, taking the title of managing partner.
It is understood that Mitchem will assume responsibility for general management of the agency, building its profile in the UK market and working on US-based Arnold accounts that will produce work through the UK office.
All three founding partners will report to Arnold's Boston-based president and managing partner, Pam Hamlin.
The appointment, which is understood to be in the late stages of contractual negotiations, will reunite Mitchem and Fourie, who worked together at Yellowhammer.
Arnold UK, which opened its doors earlier this month, is seeking a fourth and final creative partner, and an appointment is expected shortly.