Hermann August Hagen was born on May 30, 1817, in Königsberg, Prussia.

He was a German physician, entomologist, and author. He studied medicine at the University of Königsberg (M.D., 1840) and then in Berlin, Vienna, and Paris. In 1843, he returned to Königsberg, entered into the general practice of medicine, and for three years was the first assistant at a surgical hospital. He became interested in dragonflies and in 1839, he published his first paper “List of the Dragonflies of East Prussia”. Between 1855 and 1860, Hagen published several volumes of his monograph on termites "Monographie der Termiten". In 1867, he began working as an assistant in entomology at the Museum of Comparative Zoology at Harvard University and helped establish a department of entomology there. He became an assistant professor in 1868 and a full professor in 1870, the first appointment of a professor of entomology in any American institution. From 1868 to 1890, he systematically arranged and catalogued Harvard’s insect collections. He was a member of the American Academy of Arts and Sciences, the American Philosophical Society, and the American Entomological Society. He founded the Cambridge Entomological Club. In 1863, he received the honorary degree of Ph.D. from the University of Königsberg.

In 1851, he married Johanna Maria Elise Gerhards (1832-1917). He died on November 9, 1893, in Cambridge, Massachusetts.