Clifton will take responsibility for "leading and building the Mission brand through its next phase of growth and delivering sustainable shareholder value".
The former CEO of Mission ad agency Bigdog will work closely with the agency heads, which, in the comms space, includes Speed and Solaris Health, to facilitate group strategies for growth, including further cross-agency collaboration.
Clifton has worked with various agencies within the Omnicom and WPP global networks before founding Bigdog in 2008. The agency was acquired by Mission in 2012 and Clifton joined the board later that year.
"We have been through a period of significant growth, expanding through complementary acquisitions, broadening our range of services, geographic spread and sector expertise," Mission chair David Morgan said.
"James’s appointment as group CEO will ensure that we are able to both capitalise on the success that we’ve achieved and continue to build one of the most dynamic and successful groups in the UK.
He continued: "He is a strong leader, with his effective stewardship of Bigdog seeing the agency achieve excellent client retention, a series of significant business wins and a record trading year in 2018. I’m immensely proud of everything we’ve accomplished to date and I’m looking forward to working with James to take our success to the next level."
Aside from solid revenue growth, Mission has also improved its operating margin by 12.6 per cent and its profit before tax by 22 per cent to £9.5m in its most recent fiscal year.
Clifton said: "It is well documented that we are in uncertain times, especially in the advertising industry. But while other groups have been struggling, 2018 saw Mission deliver its eighth consecutive year of growth.
"Thanks to our unique proposition and quality of talent, I believe we have the opportunity to significantly increase in size, reach and capability in the coming years."