Falk Erich Walter Harnack was the younger son of painter Clara Harnack, née Reichau, and literary historian Otto Harnack; a nephew of theologian Adolf von Harnack and Erich Harnack, professor of pharmacology and chemistry; the grandson of theologian Theodosius Harnack and the younger brother of jurist and German Resistance fighter Arvid Harnack. He was also a cousin of theologian Dietrich Bonhoeffer and Ernst von Harnack, who, like his brother and sister-in-law, Mildred Harnack, also became victims of the Third Reich.[1][note 1] He never got to know his father, who committed suicide in 1914.