Born in Charlottenburg (today part of Berlin), Erwin Planck was the fourth child of theoretical physicist Max Planck (1858–1947) and his first wife Marie, née Merck (1861–1909). His father held a professorship at the Berlin Frederick William University since 1889 and had become a notable member of the German Physical Society (DPG); his children grew up in the wealthy environment of the Grunewald mansions colony.
Having obtained his Abitur degree in 1911, Erwin Planck pursued a career as an officer in the German Army. In World War I, he rather quickly found himself a prisoner of the French forces in 1914. Planck returned to Germany in 1917 and was active on the General Staff. Here he first met Major Kurt von Schleicher, the beginning of a lifelong friendship.
After the war, Major Schleicher became head of the political department in the newly established Ministry of the Reichswehr and in 1920 appointed Planck his assistant. In January 1924 Schleicher sent him as a liaison officer to the Reich Chancellery, where Planck also became a government official after he left the Reichswehr armed forces in 1926. Upon the downfall of the Brüning government in 1932, he became a deputy minister under Chancellor Franz von Papen, and retained this post when Schleicher himself was appointed Reich Chancellor in December. Planck contributed to Schleicher's Querfront to gain Nazi support for his government, though to no avail.
After Schleicher resigned and Adolf Hitler was appointed Chancellor on 30 January 1933, Planck immediately resigned office and, put in temporary retirement, went abroad to East Asia for a year. Shortly after he came back to Germany, Kurt von Schleicher and his wife were shot at their Babelsberg home by members of the Sicherheitsdienst (SD) in the Night of the Long Knives on 30 June 1934. Planck tried in vain to get an explanation for his friend's murder. He also held several conspirative talks with Reichswehr General Werner von Fritsch which, however, did not lead to any conclusion.
In 1936, Planck changed career paths and went into business, becoming a leading employee at the Otto Wolff coporate group, a large conglomerate in Cologne. In 1939, he took over leadership of the Berlin branch office.
In August 1939, on the eve of World War II, a group including Prussian Finance Minister Johannes Popitz, Planck, and Reichsbank president Hjalmar Schacht approached General Georg Thomas, head of the Defence Economy and Armament Office asking him to thwart the outbreak of the forthcoming Invasion of Poland. Thomas agreed to write a memorandum to his superior, OKW Chief Wilhelm Keitel, in which he stated that a war against Poland would set off a world war that Germany could not win owing to massive supply problems. However, Keitel tried to allay Thomas's fears by telling him that Hitler was planning no such war.
In 1940, Planck, Popitz, Ulrich von Hassell and General Ludwig Beck drafted a "Provisional Constitution" on the assumption that the forthcoming attack on the Western Front would overthrow Hitler's regime. Even afterwards, Planck stayed in the conservative resistance circles around Carl Friedrich Goerdeler and was involved in the preparations of the 20 July plot. This led to his arrest on 23 July 1944, after which he was taken to the Gestapo headquarters at the Reichssicherheitshauptamt (RSHA).
Erwin Planck was sentenced to death in a show trial at the "People's Court" (Volksgerichtshof) on 23 October. His famous father vainly asked Hitler for mercy. Planck was executed by hanging on 23 January 1945 at Plötzensee Prison in Berlin.
A memorial plaque dedicated to Erwin Planck and his fellow students Paul von Hase and Ernst von Harnack can be found at his old school, the Joachimsthalsches Gymnasium in the Berlin borough of Wilmersdorf.