Hagedorn was born in New York City and educated at The Hill School.
He graduated from Harvard University, where he was awarded the George B. Sohier Prize for literature.
[2] Though Hagedorn met Theodore Roosevelt in 1912, he visited Sagamore Hill in 1916 and became a friend.
Drawing upon his friendship with Roosevelt, Hagedorn was able to elicit the support of Roosevelt's friends and associates' personal recollections in this book, first published in 1918 and then updated in 1922.
[3] In addition to Hagedorn's works on Roosevelt and the Roosevelt family, Hagedorn also authored books of poetry and many biographies throughout his long career, including works about Leonard Wood, Edwin Arlington Robinson, and Albert Schweitzer.