Williamson, originally an Anglican, was received into the Catholic Church in 1971. After a few months as a postulant at the Brompton Oratory, he left. He became a member of the Society of Saint Pius X, a traditionalist Catholic faction founded in 1970 by Archbishop Marcel Lefebvre in protest against what Lefebvre saw as the liberalism of the Second Vatican Council. Williamson entered the International Seminary of Saint Pius X at Écône, Switzerland, and in 1976 he was ordained a priest by Lefebvre.