For many Middlemas has been the outward face of UM London for the last decade, leading its marketing, new business, PR and social media activities.
Middlemas confirmed he is leaving but declined to comment further, saying he would be with Mediabrands until well into the new year.
The 41 year old has worked his way through the agency from media planner to managing partner in 2007, and has been acclaimed for his work with major blue chip clients including MasterCard, Telegraph Media Group, Bacardi-Martini brands and Xbox.
UM and Mediabrands have been through a period of transition over the last year, with the exit of senior leaders at sibling agency Initiative and the promotion of chief executive, Andy Jones to chief executive of IPG Mediabrands.
Middlemas has been a strong advocate of the value of media collaboration and has also led UM London’s content marketing initiative, Curiosity Works.
Prior to joining UM Middlemas worked for three years at what was then BLM Media, now Arena.