William Clark was born in Aberdeen, Scotland, and was educated at King's College, Aberdeen, and at Oxford. He became a priest and, in 1882, he immigrated to Canada. He was a professor of mental and moral philosophy at Trinity College, Toronto from 1883 to 1908. Noted as a lecturer and preacher, Clark was described as one of the foremost theologians in North America. He was the author of several biographical and theological works, and became a Fellow of the Royal Society of Canada.
Mackenzie King was not one of Clark's students, but he became a good friend of Clark and his wife. In the 1930s, when King became interested in spiritualism, Dr. Clark was one of the people who spoke to him in seances.