He was born in New York City and educated at Harvard University, where he was awarded the George B. Sohier Prize for literature, the University of Berlin, and Columbia University. From 1909 to 1911, he was an instructor in English at Harvard.
Hagedorn was a friend and biographer of Theodore Roosevelt. He also served as Secretary and Director of the Theodore Roosevelt Association from 1919 to 1957. Drawing upon his friendship with Roosevelt, Hagedorn was able to elicite the support of Roosevelt's friends and associates' personal recollections in his biography of TR which was first published in 1918 and then updated in 1922 and which is oriented toward children. The book has a summary questions for young readers at the end of each chapter. Drawing on the same friends and associates of Roosevelt, Hagedorn also published the first serious study of TR's experience as a rancher in the Badlands after the death of his wife and mother in 1884. Hagedorn's access to TR's associates in these two books has been utilized by historian, Edmund Morris in his two highly acclaimed biographical books on Roosevelt published in 1979 and 2001.
Among other works, Hagedorn published:
- The Silver Blade (1907)
- The Woman of Corinth (1908)
- A Troop of the Guard, and other Poems (1909)
- Poems and Ballads (1912)
- Faces in the Dawn (1914)
- You are the Hope of the World (1917, 1920)
- Theodore Roosevelt (1918, 1922)
- That Human Being, Leonard Wood (1920)
- Roosevelt in the Badlands (1921)
- The Magnate: William Boyce Thompson and his Time (1935)
- Sunward I've Climbed, The Story of John Magee, Poet and Soldier, 1922–1941
- The Bomb That Fell on America (1946)
- "Prophet in the Wilderness: The Story of Albert Schweitzer (1947)
- "The Roosevelt Family of Sagamore Hill" (1954)
This article incorporates text from a publication now in the public domain: Gilman, D. C.; Thurston, H. T.; Moore, F., eds. (1905). "article name needed". New International Encyclopedia (1st ed.). New York: Dodd, Mead.
- Works by Hermann Hagedorn at Project Gutenberg
- http://library.syr.edu/digital/guides/h/hagedorn_h.htm Biographical History of Herman Hagedorn