He was born at Remscheid, and studied at the universities of Bonn, Heidelberg, Munich, Berlin, and Paris.

His La Walhalla y las glorias de Alemania ("Valhalla and the glories of Germany," 1872-87) performed a reverse service, describing for Spanish benefit, under the guise of interesting essays, prominent German characters from the days of Hermann.

Numerous other original volumes and translations have in a scholarly manner familiarized Germans with much of Spanish literature and history.

Fastenrath married Hungarian Luise (or Luisa) Goldmann (1858-1914), sister of writer Anna Forstenheim, a person who maintained the interests and memory of her husband.

In 1909, with the legacy of her husband, she instituted two foundations that bear her name, the "Fundación Premio Fastenrath" and the "Premi Fastenrath", which reward writers for creative works in Spanish language and Catalan language, respectively.