Clarke is leaving his regional position to take up the role of CEO at GroupM Thailand, replacing Stephanie Bell who left the agency last week. Thailand has been Kevin's base for many years and he takes on the new assignment based on his desire to return to a local market role.
Crowther joins MediaCom from the US, where he operated as CEO/ president for the Americas and Australasia at Omnicom-controlled integrated network DAVINCI Selectwork, which he also co-founded.
On his appointment, Crowther said: "I am extremely happy to be a part of the MediaCom family during this exciting period in our development and growth. With Kevin's fine leadership until now across the region, the great team we have and a vision I absolutely believe in, we will be unstoppable."
Crowther will work across 21 MediaCom offices in the region.
"Kevin has done a fantastic job at MediaCom over the past five years, turning what was a small agency into one of the region's major media agency networks that now serves many of the world's largest advertisers," said Stephen Allan, MediaCom Worldwide's chairman and CEO.
"Our business' success in the region is a testament to Kevin's leadership. I am confident that in Alex we have found a worthy successor to Kevin, someone who will continue our drive to make MediaCom the number one agency network across Asia Pacific."
Clarke joined MediaCom in 2005. Prior to that appointment he was CEO at Mindshare Thailand.
On Clarke's return to Thailand, Mark Patterson, CEO of GroupM Asia Pacific, said: "Kevin is the ideal man to lead our whole Thai operation. It is a big business with a bright future under Kevin's leadership. He has lived in and loved Thailand for more than 12 years, and led Mindshare there for eight of those. I couldn't think of a better leader."
Clarkes departure marks the second change in regional leadership at GroupM in the past month. At the start of June, MEC announced that Joost Dop would be stepping down as Asia-Pacific CEO in September this year, to be replaced by current CEO for South and Southeast Asia Stephen Li.