He has been tasked with delivering "effective, creative and best-in-class" media planning, and will report into Dan Hagen, the chief strategy officer.
Cadman will work with the heads of innovation, digital and insight to help build the agency.
He has been at MEC for more than six years where he led Johnson & Johnson’s strategic planning across Northern Europe. Cadman was seconded to J&J as the maternity cover for the head of media for Northern Europe.
Prior to MEC he was a planning director at MEC Global Solutions for three years, and a media manager at OMD for four years.
Hagen said: "JP’s in-depth understanding of clients’ needs combined with his strong agency-side background will help us drive excellence at scale across the agency.
"His UK and European experience will be extremely valuable in helping us deliver business value through media to our clients in an increasingly globalised world."
Cadman joined the agency this week. He said: "I am really excited to join an agency that I’ve always admired and respected for both the work they deliver and their new business record.
"Carat has a strong leadership team shaping the future of the agency and I am pleased to be onboard to help them deliver their vision."