A native of New York City from birth, Hagedorn studied at Harvard, the University of Berlin, and Columbia University. He taught in the English Department at Harvard for a few years before he began to write full-time. In addition to poetry, Hagedorn wrote biographies and novels.
Hagedorn met Theodore Roosevelt in 1916 following his time in office. He was captivated by Roosevelt, and after the two became friends, Hagedorn asked Roosevelt for his help writing The Boys' Life of Theodore Roosevelt, which was published in 1918. That volume is especially oriented toward children.
Following Roosevelt's death in 1918, Hagedorn became the executive secretary of the Roosevelt Memorial Association. He wrote several more books about Roosevelt, as well as biographies of figures who were close to Roosevelt (including poet Edwin Arlington Robinson).