He starts on 1 August and replaces Jonathan Shore, who has been interim CEO of GCI London alongside his UK CEO role at WPP sister agency Cohn & Wolfe.
Shore's unorthodox double-reign now ends, as he returns to C&W full-time. Cater's hire would seem to signal WPP's commitment to the GCI brand after months of speculation over its future. GCI's previous full-time UK CEO was Adrian Wheeler, who left last year.
Cater's healthcare pedigree is significant given that GCI's UK health arm closed after MD Rhonda Smith left.
Its UK decline in healthcare has been embarrassing for an agency with major global clients in the sector. Cater said resurrection of GCI's UK healthcare arm was a priority but that he aimed to boost the agency, which is believed to have around 75 staff, across the board.
He particularly wants togeographically extend the remits it currently handles for non-UK clients.
Cater left Ketchum in February to join APCO Worldwide as director of strategic comms, healthcare. He left after three months.
At GCI, Cater - reporting to US-based president/CEO Jeff Hunt - is part of a senior UK management triumvirate alongside deputy MD Rhodri Harries and operations director Ginni Arnold.