Cartwright joined Ketchum in 2008, having previously managed the UK public affairs teams at Hill+Knowlton Strategies and GCI Health.
He took on his current role in the summer of 2011, succeeding Rob Flaherty - who is now the firm's CEO - and was made a partner in 2012.
Cartwright said: "Moving forward, I'm looking forward to pinpointing the right opportunity to do what I love - to lead, to counsel and to create."
As director of Ketchum's global corporate practice, he has overseen specialists in corporate media relations, public affairs, financial communications, issues & crisis, CSR & sustainability and energy, according to the firm's website.
The agency did not immediately name a replacement, telling PRWeek it is now considering how best to structure its corporate team leadership.
Denise Kaufmann, CEO of Ketchum London, said: "As our global corporate practice continues to grow and evolve, Rod Cartwright has made the decision to pursue the next stage of his career outside of Ketchum. He has made an exceptional contribution to Ketchum over the last nine years. We are grateful for all his work and wish him the very best for the future."
Cartwright was a founding director of the now defunct UK Public Affairs Council, has lectured at Imperial College London and sat on the board of UK trade body the PRCA. He is one of the mentors in this year's PRWeek/PRCA Fast Track mentoring scheme.