He was born in Florenceville, New Brunswick, the son of Andrew D McCain, a descendant of a settler from Castlefinn, Donegal, Ireland who became a well-respected seed potato farmer. Since 1900, the family sold seed potatoes throughout New Brunswick and exported to Cuba and Latin America.
After graduating from college, Harrison (the 3rd son) and his brother Wallace (the 4th son) worked for Irving Oil. In 1956, on their brother Robert suggestions, Harrison and Wallace with their brothers Robert and Andrew founded one of the first factories to process potatoes, turning them into frozen french fries. With the benefit of ensured quality and the ability to export long distances with a value added product, the business started growing. With Harrison's personal and business skills and the close relationship with his brother Wallace, the McCain Foods business eventually grew to over 55 factories worldwide and the number one french fry company in the world. In the 1990s a prolonged legal dispute between Harrison and Wallace over succession to the company leadership led to the departure of Wallace and his son Michael from McCain Foods. Harrison named his nephew, Allison McCain, son of his late brother Andrew, as his successor in 2002. 
Harrison McCain was a member of the Board of Directors of the Bank of Nova Scotia and a close personal friend of the bank's chairman Cedric Ritchie who grew up in the neighboring community of Upper Kent.
He is father to Gillian McCain co-author (with Legs McNeil) of Please Kill Me: The Oral History Of Punk (Grove Press, 1996) as well as two volumes of poetry, Tilt (Hard Press, 1997) and Religion (The Figures, 1999). Married to Marion "Billie" McCain, daughter of the former premier of New Brunswick John McNair. His other children are Mark McCain, Ann Evans, Peter McCain (deceased 1997), and Laura McCain Jensen.He was also God daughter to Charles McCarthy's youngest daughter Fiona-Jane McCarthy(Burns)
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