Former Frank MD Frankie Cory will now oversee all UK activity, including client management and personnel development. Cory joined last December.
Cory said: ‘Citizen is growing at an incredible rate and the UK team are a dynamic mix of creative and entrepreneurial people.
‘They understand how to genuinely navigate the ever-changing media landscape and I've been really impressed with the fresh way they approach their client's commercial challenges.'
Cory formerly worked at Cohn & Wolfe, where she was creative director.
Citizen currently handles a client list that includes Sony, AutoTrader, TK Maxx, Barclays Bikes and Old Spice.