Archbishop Duncan was born on July 5, 1948, and raised in Bordentown, New Jersey. The eighteenth priest (and second bishop) to be ordained from Christ Church in Bordentown, Bishop Duncan graduated as valedictorian of the Bordentown Military Institute in 1966. He graduated with honors from Trinity College in Hartford in 1970 and from the General Theological Seminary in 1973. He also undertook advanced research in Scottish History at Edinburgh University in 1972-73. Sabbatical study in 1987 focused on "Healing and Miracles in their Biblical, Historical and Contemporary Contexts." Sabbatical leave in 1997 focused on "God’s Vision for the Diocese." In 2003, Archbishop Duncan was Bishop in Residence of Nashotah House and did sabbatical study in Liturgics and French Conversation. Ordained deacon on April 22, 1972 during his Middler Year at General Seminary, and priested on the Feast of Ss. Simon and Jude the following October, Archbishop Duncan served the Chapel of the Intercession in Manhattan, Christ Church in Edinburgh, Scotland, and Grace Church in Merchantville, New Jersey, during his first years in holy orders. He was assistant dean of General Seminary from 1974-78, Episcopal chaplain of the University of North Carolina at Chapel Hill from 1978-82, and rector of Saint Thomas's Parish in Newark, Delaware, from 1982-92. In 1992, he became canon to the ordinary for Bishop Alden Hathaway in Pittsburgh.

Archbishop Duncan has devoted himself to mission and evangelism throughout his years of ministry, with a special passion for reaching adolescents and young adults. He has also led short-term missions in Haiti, Trinidad, and Rwanda. He is a champion of the poor and dispossessed of the Diocese of Pittsburgh, encouraging creative urban church-planting, as well as many other church-plants. He is active in the Cursillo and Happening movements and served as an elected member of the General Board of Examining Chaplains of the Episcopal Church from 1991 until 1997. Archbishop Duncan is a trustee of Trinity Episcopal School for Ministry and of Nashotah House.

Recognized internationally for his commitment to capacity-building in the two-thirds world, Archbishop Duncan served on the Programme Committee of the Network for Anglicans in Mission and Evangelism, an agency created at the 1998 Lambeth Conference. In 2004 Archbishop Duncan was a driving force in the creation of the Anglican Relief and Development Fund, a multi-million dollar enterprise for which he continues to serve as President.

Best known beyond Pittsburgh for his role as Moderator of the Anglican Communion Network (2003-2009), and as Chairman of the Common Cause Partnership (gathering eleven orthodox Anglican bodies in the United States and Canada) from 2004 until the inception of the Anglican Church in North America in 2009, when he became Archbishop and Primate, Bishop Bob (as he is known globally) has proved himself an extraordinary leader, as well as a "brave heart," at a critical moment in Church history.

Archbishop Duncan married Nara Dewar on August 16, 1969. They are a dynamic team. They share a love of gardening, travel, hospitality and music. They have one married daughter, Louise Elizabeth, for whom, with her husband Mark, two grandchildren and their numerous nieces, nephews and godchildren, they are intensely thankful.

God's vision for the Diocese of Pittsburgh under Bishop (now Archbishop) Duncan is "One Church of Miraculous Expectation and Missionary Grace." The vision is accomplished in five areas: building congregations, making disciples, establishing partnerships, gathering resources and recruiting youth. God’s vision for the Anglican Church in North America is of a biblical, missionary and united Church. The articulated mission is "to reach North America with the transforming love of Jesus Christ." When successes, triumphs or challenges are pointed to, the Archbishop's response is predictable: "It's 90% prayer, 10% all the rest."