Bishop Duncan was ordained Deacon in 1964 and Priest in 1965. His first appointment from 1965to 1969 then the Fairford First Nations Mission. He then served in the Winnipeg parish of St. Anne's (1969-1974) where he became Priest Associate in the River North Anglican parishes, a team ministry involving five parishes (including St. Anne's). From 1974 to 1978 he was Rector of Grace Church, Milton in the Diocese of Niagara.

In 1978, Bishop Duncan was appointed Rector of St. Paul's Cathedral, Regina and Dean of the Diocese of Qu'Appelle. In this position he chaired numerous Diocesan committees; acted several times, for the presiding bishop, as Diocesan Administrator; was a delegate to the provincial Synod of the Province of Rupert's Land and a delegate to the General Synod of the Anglican Church of Canada, where he served as a member of two key General Synod committees, the National Executive Council and Doctrine and Worship Committee.

He was also Canadian representative to the Executive Council of the Episcopal Church (USA). On November15, 1997, Duncan Douglas Wallace was elected as the 10th Bishop of Qu'Appelle. Bishop Wallace retired in 2005.

Bishop Duncan is married to Mary Emily, a school principal with the Regina Public School Board. They have two children, daughter: Lisa Wallace (Mike) Markesteyn. Lisa and her husband are teachers living in Regina and are the proud parents of the bishop's only grandchild, Drew, born February 22, 2000. Bishop Duncan and Mary also have a son, Andrew (Rayna) who is an Urban Planner living in Calgary.

In his spare time Bishop Duncan enjoys choral and piano music (he is an accomplished pianist), sailing, skiing, squash, cooking and renovation projects. Bishop Duncan has a keen interest in music and worship and in the role of Cathedrals in the Anglican Church. St. John's College in Winnipeg, his alma mater, conferred an honorary Doctorate of Divinity on him in 1993.