[3][4]) He began his baseball career at St. Peter's Preparatory School in Jersey City, which inducted him in 2005 into the first class of its athletic hall of fame.

[7] After posting a 17–7 win–loss record and leading the Class D New York–Penn League in strikeouts, he was selected by Washington in the first-year player draft.

[3] After the 1970 campaign, the Senators traded Hannan, fellow right-hander Joe Coleman, shortstop Ed Brinkman, and third baseman Aurelio Rodríguez to the Detroit Tigers for former 30-game-winner Denny McLain, Don Wert, Elliott Maddox, and Norm McRae.

[8] Hannan only got into seven games for the 1971 Tigers and was effective in middle relief, before being traded to the Milwaukee Brewers on May 11 for John Gelnar and José Herrera.

He compiled a 41–48 record, with a career ERA of 3.88, allowing 807 hits and 408 bases on balls in 822 innings pitched.