 In 1916, he vigorously defended Catholic institutions in New York during their investigation by the Charities Department under Mayor John Purroy Mitchel; he was charged with crimes against the city, including libel, obstruction of justice, and perjury based on information obtained through wiretaps, but were later dropped.On August 19, 1921, Dunn was appointed Auxiliary Bishop of New York and Titular Bishop of Camuliana by Pope Benedict XV.He received his episcopal consecration on the following October 28 from Archbishop Patrick Hayes, with Bishops Joseph Conroy and John O'Connor serving as co-consecrators, at St. Patrick's Cathedral.