Today mostly forgotten, André Mandelstam (1869-1949) was a pioneer of the human rights movement in the interwar period. Originally a diplomat in the service of the Russian Empire, he went into exile after the Bolshevik revolution and became an important member of the internationalist scene in Paris. An active contributor to the various professional associations and institutions of the time, Mandelstam came to draft the first ever international human rights declaration which was pronounced by the Institut de droit international at its New York session in 1929. His work on human rights protection was influenced by his experiences as diplomat in Constantinople where, in the years preceding the First World War, he had witnessed the growing tensions over the treatment of the Armenian population of the Ottoman Empire. This article traces Mandelstam’s impact on the development of international human rights law and uncovers the driving forces for his work: the end of the Russian and Ottoman empires as well as his career change from diplomat to academic activist. The contribution invites us to reconsider traditional narratives of the origins of international human rights protection as well as to rethink the imperial(ist) influences upon this development.