John Treacy, executive creative director at Proximity, is stepping down after the agency merged with Rapp last month.
Treacy, who has worked at Proximity since 2013, said he is taking the opportunity "to go in a different direction".
He joined the agency from Elvis, where he spent 10 years. Treacy has also worked at Archibald Ingall Stretton.
The newly merged Rapp UK will led by chief creative officer Al Mackie, who joined the business in 2017. He was previously at Rapier for two years, having spent the previous four years at Lida.
Treacy said: "As I look back, it’s been an amazing ride, but I’m taking the opportunity to now go in a different direction. After nearly seven years at Proximity and 18 years as a CD/ECD, it’s time for a break.
"I know you’ll all make a massive success of the merger and I want to thank everyone past and present who have helped make Proximity such a special place."
Omnicom dropped the Proximity name and folded it into Rapp in early May. The new UK entity is being led by Chris Freeland as executive chairman and Gabby Ludzker as chief executive.
Ludzker said: "JT smashed the brief we gave him at the start of his career at Proximity – to get back in front of juries – by winning nearly 200 creative awards globally, including a Cannes gold Effectiveness Lion.
"JT found and nourished an incredibly talented team who we cherish. We’re sad to see him go, but can’t wait to see what he achieves next – no doubt gold dust!"