The company is best known for having the world's largest advertising billboard - at Fort Dunlop, near Birmingham - which recently featured an Adidas poster with a 75-foot high picture of David Beckham's head.
Hopkinson's task will be to drive the company's development in the UK and internationally. He will also help promote Retail Profile, its sister operation, which specialises in leasing and selling promotional space in shopping malls.
Hopkinson, 44, who had been running AMV's ICI, Highland Distillers and Guinness accounts, quit the agency in May after 17 years.
Mega Profile claims a 70 per cent share of the UK giant poster market, which is estimated to be worth £10 million this year. The company was established in 1997 to specialise in giant posters on landmark construction sites in major cities across the UK. Recently it set up the Mega Group in association with leading giant poster contractors across Europe to offer advertisers international campaigns.
Hopkinson said: "It's a wonderful challenge, an opportunity to take an active part in the company at a fascinating stage in its growth and to be able to contribute to its international development."
Simon Evans, Mega Profile's chief executive, said: "We're delighted to have Andy on board. He obviously has deep commitment to the company and his experience and knowledge will be invaluable."