He was employed by Lionel Walter Rothschild as ornithological curator of his private museum at Tring from 1892 to 1929. Hartert published the quarterly museum periodical Novitates Zoologicae (1894-1939) with Rothschild, and the Hand List of British Birds (1912) with Francis Charles Robert Jourdain, Claud Ticehurst and Harry Witherby. He wrote Die Vögel der paläarktishen Fauna (1903-1922) and travelled in India, Africa and South America on behalf of his employer.
In 1930, Hartert retired to Berlin, where he died.
A species of lizard, Hemiphyllodactylus harterti, and 12 birds are named in his honor.
Zoological Society of London.