According to Media Square, MacKenzie's additional media commitments have forced him into stepping down from the role.
Jeremy Middleton, chief executive of Media Square, said: "We're grateful to Kelvin MacKenzie for all his hard work on the group's behalf during the last 12 months.
"Kelvin provided stalwart support during a difficult period in the group's development. We wish him well with the new ventures he's currently involved in."
MacKenzie, who has been outspoken on many subjects over the years, including the Hillsborough disaster, immigration, life expectancy in Scotland and the government, occasionally appears on BBC Radio 5 and has a regular column at his former title, The Sun.
Media Square's range of businesses include sales promotion agency Dynamo, digital agency Twentysix London and direct marketing agency CMW.