Cornelius James Brosnan was born in Michigan on June 14, 1882. The Brosnan family moved to Idaho in 1912, where he served as superintendent of schools in Nampa, then as the head of the history department in Boise High School for several years. From 1920-1921 he was an Austin Scholar at Harvard University, and after receiving his M.A. from that institution in 1921, he returned to Idaho as a member of the history faculty at the University of Idaho. He received his PhD from the University of California, Berkeley. In 1930, he became the head of the Department of American History.

In addition to teaching at the University of Idaho, he also did extensive historical writing on the northwest and was nominated for the Pulitzer Prize in 1932. Dr. Brosnan retired from the university in 1952 and became Professor of American History Emeritus.