GA said Millard, who headed College Hill's business development and marketing functions but did not hold equity, would manage the firm's corporate team and report to London CEO Fergus Wylie.
Millard has formerly been a director with Brunswick, having joined in 1988 from Alton Towers, where he was corporate affairs director. He left Brunswick in 1990 to help set up C&W's London office.
He joined College Hill in 1992 as a director and became equity partner under chairman Alex Sandberg. Millard ran College Hill's corporate comms and crisis management teams but left at the end of 2002 to move to Australia. He then spent one year in Sydney as MD of Australian corporate communications firm Cannings.
Millard returned to the UK last year to rejoin College Hill and worked on the firm's property, recruitment and transport accounts. He told PRWeek he was 'sad' to be leaving the agency.
Sandberg was not available for comment.