He will report to Brussels-based Brad Staples, APCO chief executive across EMEA.
Cater expects to spend around two thirds of his time in Europe and the rest in Asia and North America.
APCO handles PA for various pharma firms, but has little heritage in product comms.
Cater said: 'APCO's offering will address a market need and provide a powerful alternative to most agencies.'
He leaves Ketchum – where his clients include Roche, Novartis and GlaxoSmithKline – next week and starts at APCO on 27 February. He was promoted to his current role ten months ago.
Cater first encountered Staples, and other APCO staff, four years ago when they both advised on business for Swiss firm Roche.