Zacharias spent the summer of 1971 in South Vietnam, where he evangelized US soldiers, as well as imprisoned Viet Cong members. After graduating from Ontario Bible College, he began an itinerant ministry with the Christian and Missionary Alliance (C&MA) in Canada. In 1974 the C&MA sent him to Cambodia, where he preached only a short time before its fall to the Khmer Rouge. He was later ordained by the C&MA in 1980, and between 1980 and 1984, he taught at the C&MA-affiliated Alliance Theological Seminary, where he was a professor of evangelism.