The former Sun editor and TalkSport chief executive will help set the future strategy of the recently expanded company.
The appointment follows Media Square's £63 million acquisition of the marketing services businesses of Huntsworth plc in late 2005. The deal included the acquisition of companies including sales and promotions company Dynamo market research firm Incepta Marketing Intelligence and online advertising and marketing company Incepta Online.
Jeremy Middleton, chief executive of Media Square, welcomed MacKenzie's appointment: "Kelvin's experience of the media sector and his general management expertise are of considerable value to the group. He will be working with us to reinforce Media Square's position as a significant player in the marketing and communications area."
MacKenzie said: "The Media Square story is fascinating, but what I have found most compelling is the adeptness at which the team there has created a top-five quoted marketing group, with over 2,500 clients, in just three years."
MacKenzie left his position at Highbury House after an attempt to save the company from financial collapse.
The publisher was put into administration with £27m debts last Friday, and its magazines have since been sold to a number of other publishers.