Johnson will lead the sales team in the UK and help drive ad revenue and growth across The Huffington Post UK, MSN, Skype, Engadget, XBox and TechCrunch.
He has been tasked with growing AOL’s overall mobile revenues in the UK.
Johnson will report to Stuart Flint, who was promoted to managing director after Hamish Nicklin left to join The Guardian as chief revenue officer.
Johnson joined Millennial Media in November 2014. Before this, he was the vice president of UK sales at Specific Media for nine months, having joined the company as head of UK sales in 2013.
He has also worked at Virgin Media and ITV.
Flint said: "AOL is already making significant headway in 2016, disrupting the status quo within the industry.
"Our mission is to be the leading mobile media technology company, and Gavin’s great experience across mobile, broadcast and digital advertising make him the ideal candidate to lead our established sales team."