Tim Pearson, chief executive of Manning Gottlieb OMD, has been promoted to an expanded role as chief executive of OMD Group UK.
Pearson, who has worked at Manning Gottlieb OMD for more than 20 years, will oversee both sister agency OMD UK and his existing agency in his new job.
He has taken the bigger role as part of wider shake-up as Dan Clays, chief executive of OMD UK since 2015, becomes UK chief executive of parent company Omnicom Media Group UK.
Pearson, who has led Manning Gottlieb OMD since 2015, will report to Clays.
Omnicom Media Group said Manning Gottlieb OMD and OMD UK will retain "separate leadership teams" and there are no plans to merge them.
It is expected that Manning Gottlieb OMD will continue to be led by managing director Natalie Bell and Paul Knight, chief executive of OmniGov, the specialist unit that handles the UK government’s media buying.
Similarly, OMD UK is set to keep Laura Fenton as managing director and Jess Roberts as chief client development officer.
Pearson has proved a strong leader of Manning Gottlieb OMD, winning the UK government’s £150m media buying account last year, and it was named Media Agency of the Year by Campaign in March.
He said: "These are exciting times for media agencies and the opportunity to step up and assume responsibility for the entire OMD UK Group portfolio is one I’m going to relish.
"Both agency brands are strong and will continue to be independent of each other.
"I look forward to rising to the challenges this enhanced responsibility presents, to getting to know everyone at OMD UK and to working closely with Dan to drive further success."
Manning Gottlieb OMD is located in Bankside on the south side of the River Thames in London. OMD UK is based north of the river in Fitzrovia.
OMD Group UK has previously had a chief executive but the role has been vacant since Steve Williams departed in 2012 to become president at PHD New York. Williams later joined WPP.
OMD UK and Manning Gottlieb OMD already report financial numbers together as OMD Group UK.
The group has 864 staff and billings of £800m, according to OMD Group UK’s latest Campaign School Report.
Clients include Channel 4, Disney, John Lewis & Partners and Specsavers.