Millar converted to Catholicism, with his mother, at age ten. Educated for a time in Europe, it was there that he decided to enter the Jesuits. In 1920, he began teaching at Fordham University in 1920, where he spent most of the rest of his life. His main academic field was political philosophy, and he wrote a number of articles on the relation between the American founding and Catholicism, as well as serving as associate editor of the Jesuit journal Thought. He also co-authored, with John Ryan, the important text, The State and the Church (1922).