While at Fairfield, he read The Long Loneliness, the autobiography of Dorothy Day, which inspired him to join the Catholic Worker movement.
"[4] Cornell led the first protest against the Vietnam War, which started with only two people from the Catholic Worker, himself and Chris Kearns, on July 16, 1963, in Union Square in New York City.
[5] As US military engagement was intensifying in Vietnam, Cornell founded the Catholic Peace Fellowship with Jim Forest in which they worked counseling Catholic conscientious objectors to the Vietnam War, before going on to counsel anyone with draft issues with the aid of Center on Conscience & War, an organization dedicated to defending and extending the rights of conscientious objectors, claiming a "very high" success rate.
[7] In 1967, Cornell signed a public statement declaring his intention to refuse to pay income taxes in protest against the U.S. war against Vietnam.
[4] At the Fourth World Congress in 2000, he served as Pope John Paul II's deacon at a Mass of Christ the King in St. Peter's Square.