William Duncan is the director of the Marriage Law Foundation, a legal organization whose mission is providing legal resources in defense of marriage as the union of a husband and wife. He is also the director of the Center for Family and Society at the Sutherland Institute. He previously served as acting director of the Marriage Law Project at the Catholic University of Americas Columbus School of Law and a visiting professor at Brigham Young Universitys J. Reuben Clark Law School where he was executive director of the Marriage and Family Law Research Grant.

He has submitted legal briefs on behalf of pro-family organizations in many courts, including the highest courts of Alaska, California, Georgia, Massachusetts, New Jersey, New York, Washington; the U.S. Court of Appeals for the Fifth, Eighth and Eleventh Circuits; and the U.S. Supreme Court. He has drafted pro-family legislation that has been enacted in various states and has testified in front of legislative committees in Idaho, Maryland, New Mexico, Pennsylvania and Utah.

Mr. Duncan is married to Catherine Allred Duncan and they are the home schooling parents of seven children.