Viola MacMillan had two careers in the Canadian mining industry. First, over a period that spanned four decades, she and her husband teamed up as prospectors and developers of several substantial mineral deposits across the country. Her success as a mine finder and financier was substantial, but she was also the driving force behind the transformation of a small, regional association of prospectors into an internationally recognized association of professionals involved in all aspects of mineral exploration in Canada.

Viola MacMillan rose to prominence along with her husband as a syndicate-financed prospector. Under MacMillan's leadership, they became the developers and producers of both precious and base metals across Canada. In Ontario, notable contributions included the early discovery of the Hallnor deposit in the Timmins area followed by the development of the Canadian Arrow open pit gold deposit. She acquired and developed the Kam-Kotia base metal mine, also in the Timmins area, and directed the development of the silver-lead orebodies of ViolaMac Mines in the Sandon area of British Columbia. In the Beaverlodge area of northern Saskatchewan, she completed the earlier started development of the Lake Cinch uranium orebodies and placed the mine into production.

But MacMillan's greatest contribution to the industry, and one that can not easily be measured, is her driving commitment to transforming the Prospectors and Developers Association from a small group of less than 100 to a vital organization of more than 4,000. Today, the Prospectors and Developers Association of Canada is national in scope, active in helping set public policy as it pertains to the mineral industry and one of the largest of its kind in the world whose annual convention attracts international attendance.

She first served as secretary of the small and informal lobby group but was elected president just before the Second World War, serving in that capacity until 1966. During the war, she was much involved with both the provincial and federal governments in working with the wartime Metals Control Commission. After the war, when the country's gold mining sector was in danger of collapse, she was instrumental in persuading the government of Canada to introduce the Emergency Gold Measure Act, legislation that saved the stagnating gold mining industry in the 1950s and 1960s.

MacMillan was born April 21, 1903, at Windermere in the Muskoka district of Ontario, the thirteenth of 15 children. After business college, she worked as a stenographer. Her interest in mining was sparked while visiting her brother, a miner in a Cobalt silver mine, who took her underground disguised by heavy mining overalls because women were considered unlucky underground and were unwelcome.