Williams is a Presbyterian who served as a pastor for several years before becoming a full-time educator. As a key figure in the burgeoning charismatic movement of the 1960s, he was President of the International Presbyterian Charismatic Communion, and later a participant in the International Roman Catholic-Pentecostal Dialogue. In 1972 became Founding President of the Melodyland School of Theology in Anaheim, California. He joined the faculty of Regent University in the mid-1980s.
Important works include the three-volume systematic theology entitled Renewal Theology (1988-92, published as a single volume in 1996), the first complete systematic theology written from a charismatic perspective. It included chapters on the supernatural gifts of the Spirit and the a chapter on the Baptism of the Holy Spirit, arguing for the traditional Pentecostal interpretation.