Jacob Hanau studied medicine at the universities of Giessen, Berlin and Vienna, and qualified as Doctor of Medicine (MD) at the University of Giessen in 1870. His inaugural dissertation was titled Ueber die Arten von Hernien, die durch ein Hinderniss in ihrem Verlaufe gezwungen eine abnorme Richtung einschlagen (Giessen, 1870, 65p). He served as a medical assistant in the Franco-Prussian War of 1870-1871 and was awarded the Hessian Sanitary Cross.
In December 1880 Hanau was licensed to practice in the Cape Colony and settled in Carnarvon, where he became the town's first district surgeon in 1884. Soon he served also on the local Library Committee, and became one of the directors of the government-aided Undenominational School. By 1895 he was a member of the Cape of Good Hope Branch of the British Medical Association.
In 1891 Hanau donated a large collection of beetles to the Albany Museum, Grahamstown. He had by that time been collecting for several years. In 1899 he became a member of the South African Philosophical Society. However, by 1907 he had returned to Germany and was living in Wiesbaden.