 Harnack was born in Marburg as the son of the theologian Adolf von Harnack (1851–1930) and Amalie Thiersch (1858–1937), the granddaughter of the chemist Justus von Liebig.On 25 March 1916, in Hindenburg (Upper Silesia), he married Anna (Änne) Wiggert (b.5 October 1894, in Göttelborn, Ottweiler district, Saarland; d. 22 August 1960, in Berlin), daughter of the Prussian official Ernst Wiggert and Elisabeth Schmidt.