He taught economics at St. Mary's Seminary and University in Baltimore, Maryland. He was also Assistant Director of the Department of Social Action for the National Catholic Welfare Conference. Toward the end of the Second World War, Cronin wrote a report for the bishops on the Communist Party in the United States. He had the assistance of FBI officials, who unofficially provided some of the background material for him. When Richard Nixon was elected to Congress in 1946, he sought out information on Communism, and he was introduced to Cronin by Rep. Charles Kersten (R.-WI.). Although Nixon did not mention it, Cronin doubtless told him that Alger Hiss had been a member of the Communist Party. When Whittaker Chambers testified before the House Committee on Un-American Activities in August 1948 and said that Hiss was a Communist, Nixon already knew about the charge from his conversations with Cronin. After Nixon was elected vice president, he asked Cronin for help in writing speeches, and Cronin became an unpaid assistant to the vice president.
In the 1950s-1960s, he advocated increased civil rights for all Americans. He wrote two of the bishops' statements on race relations and lobbied them to see that they accepted the drafts.
He authored the book Communism: A World Menace. In addition to Communism, Cronin criticized anticommunist extremists in the United States, whom he accused of fostering national disunity.