Joan Heather Tratch was born on October 30, 1941 in Cudworth, Saskatchewan to John Tratch and Edna Blakley. She received her primary and secondary education at Wakaw, Saskatchewan and studied pharmacy for two years at the University of Saskatchewan. She married Donald John Duncan on October 10, 1964 and had four children: William, Heather, Richard, and Michael. Duncan and her husband owned and operated Duncan's Drug Store in Maple Creek.

Duncan was first elected to the Saskatchewan Legislature in 1978 and served as a Progressive Conservative MLA for the Maple Creek constituency until 1991. Duncan and Patricia Smith were the first female Cabinet ministers appointed to Executive Council in Saskatchewan. Duncan served in the Grant Devine Government as Minister of Government Services (1982-1983); Minister of Revenue, Supply and Services (1982-1983); Minister of Revenue and Financial Services (1983); Minister of Supply and Services (1983); Minister of Consumer and Commercial Affairs (1983-1988); Minister of Co-operation and Co-operative Development (1986-1987) and Minister of Economic Development and Tourism (1988-1989).

Duncan was Minister Responsible for Saskatchewan Government Printing Company (1982-1983); Board of Revenue Commissioners (1982-1983); Public Service Superannuation Board (1982-1983); Public Employees Superannuation Plan (1983); Saskatchewan Development Fund (1982-1983); Office of the Rentalsman (1983-1987) Rent Appeal Commission (1983-1987) Provincial Mediation Board (1983-1987); Saskatchewan Securities Commission (1983-1985); Saskatchewan Minerals (1983-1986); Saskatchewan Government Insurance (1986-1988); Agricultural Implements Board (1985-1988) and Saskatchewan Economic Development Corporation (SEDCO) (1988-1989).

On October 3, 1989, Duncan resigned from Cabinet and announced that she would not seek re-election. She continued to represent her constituency until Jack Goohsen (Progressive Conservative) was elected MLA for the Maple Creek constituency in October 1991.