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 elicit 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 three highly acclaimed biographical books on Roosevelt: The Rise of Theodore Roosevelt, Theodore Rex, and Colonel Roosevelt.
