After law school, he entered into private practice for several years. In 1935, he began in the New York County District Attorney's office as an Administrative Assistant District Attorney under Thomas E. Dewey. In 1941, Dewey announced that he would not seek re-election. He suggested four prosecutors whom he believed should succeed him. Although Dewey's list included Hogan's name, his selection surprised Dewey. First Hogan, unlike Dewey, was a registered Democrat and his list was meant for the Republicans. In addition, Dewey did not expect the support of Tammany Hall for his preferred candidate, after he prosecuted some of their leaders. Not only did Tammany offer its support, but the Republicans and the American Labor Party endorsed Hogan, which guaranteed his election. Thus, he became the new District Attorney on January 1, 1942.