Born in Florenceville, New Brunswick, to the owner of a seed potato exporting Business, he received a Bachelor of Arts from Mount Allison University in 1951.
McCain co-founded McCain Foods Limited in 1956 with his brother Harrison McCain, building it into one of the world's largest frozen food companies. In the 1990s a prolonged legal dispute between Harrison and Wallace over succession to the company leadership ended with the departure of Wallace's sons Michael and Scott McCain from McCain Foods. Father and son moved to Maple Leaf Foods.
In 1995, he was made an Officer of the Order of Canada for being "one of Atlantic Canada's most notable entrepreneurs" and was promoted to Companion in 2008. In 1993, he was inducted into Canadian Business Hall of Fame and the New Brunswick Business Hall of Fame in 1997. In 2003, he was awarded the Order of New Brunswick.
Wallace and Harrison were estranged after the fallout from their McCain Foods dispute. In 2004 there was rapprochement between the brothers when Wallace visited Harrison who was sick in the hospital. Wallace has also been a confidante to McCain Foods' non-family executives who now run the firm. 
McCain died on May 13, 2011 of pancreatic cancer. He was 81. He was residing in Toronto, Ontario at the time of his death.