Matthew Platts, executive director for media and performance at Dentsu Aegis Network, is leaving after 27 years at the business.
Platts joined BBJ Media Services as a buyer in 1993, rising to commercial director of Vizeum in 2007, president of Amplifi UK in 2013 and taking on his current role in 2017.
During this time, Platts was regularly named as one of Campaign's top 10 media buyers, most recently in 2017.
Euan Jarvie, chief executive of Dentsu Aegis Network UK and Ireland, said: "Matthew has played a pivotal role in shaping Dentsu Aegis Network’s UK operation into the business it is today. He’s established Amplifi UK as a true industry heavyweight and helped our media agencies forge deep, productive relationships with both our clients and media partners.
"Matthew leaves a transformed business that strives to offer our stakeholders the most seamless experience possible. We fully appreciate what he has done for us, both personally and professionally, and wish him all the best in the future."
The network is searching for a successor. It is not yet clear when Platts will be leaving the business.
Platts said: "When I joined BBJ it would have been difficult to imagine the business it would become. The fact that I have stayed here so long is testament to the challenges and opportunities the business has offered in that time.
"More importantly, it underlines that brilliant people that I have had the pleasure of working with across all areas of the industry. I’m certain Dentsu Aegis Network will continue to go from strength to strength. Now I can’t wait to take stock and embrace new opportunities outside it."
Platts' exit follows a string of senior departures, including his former boss, Jerry Buhlmann, the co-founder of BBJ Media, who stepped down as global chief executive of Dentsu Aegis Network in December 2018.
Other more recent UK departures include Pippa Glucklich, who quit as chief executive of Amplifi UK after a reorganisation of the buying arm, and Gyro chair Kate Howe, who will join MSQ Partners next month.