The grandson of Maximilian von Montgelas, he joined the army in 1879, served in the Boxer Expedition, and was Military Attaché in Peking from 1901 to 1902. In 1914 he commanded the 4th Bavarian Infantry Division, but retired the following year to devote himself to careful study of the matters relating to the outbreak of World War I and responsibility for it. In this capacity he was official adviser to the Reichstag Committee of Enquiry.
He was a German count and official spokesman for the German Weimar Republic at the Paris Peace Conference following World War I to investigate the question of responsibility for the war. He helped to draft the German answer to the charges of war guilt and was one of the four signatories to the Memorandum, presented on May 29, 1919, in reply to the Western Allies. Later, in the absence of the other members of the German Commission, he was jointly responsible, with Delbruck, for a further Memorandum replying to the Allied Note of June 16, 1919.
He subsequently wrote his controversial book The Case for the Central Powers: An Impeachment of the Versailles Verdict published in London by George, Allen & Unwin Ltd., in 1925. The previous year he and Professor Walther Schucking had edited The Outbreak of the World War - German Documents collected by Karl Kautsky (commonly known as the Kautsky Documents) which were published by the Oxford University Press.