Born in Bloomington, McLean County, Illinois, Brennan attended parochial schools. He was employed as a reporter for the Bloomington Bulletin. He was graduated from the Illinois Wesleyan University in Bloomington, Illinois, in 1902.
Brennan's early life was marked by his education at parochial schools in Bloomington, Illinois. He went on to work as a journalist for the Bloomington Bulletin before graduating from Illinois Wesleyan University in 1902.