Lossow was born in Hof in the Kingdom of Bavaria. He entered the Bavarian Army in 1888. He served in a variety of assignments, and was trained as a general staff officer. He served with the German contingent of the relief expedition during the Boxer Rebellion. Immediately prior to World War I, Lossow was a lieutenant colonel and a general staff officer without a specific assignment. On mobilization in August 1914, he was assigned to be the chief of the general staff of the II. Bavarian Reserve Corps. Lossow served with the corps until July 1915, when he became the German military attaché in Istanbul in the Ottoman Empire, where he assisted the Ottoman Army and the German military mission in planning the ongoing response to Allied landings in Gallipoli. He also provided valuable testimony concerning the Armenian Genocide in its later stages, in which he wrote that "On the basis of all the reports and news coming to me here in Tiflis there hardly can be any doubt that the Turks systematically are aiming at the extermination of the few hundred thousand Armenians whom they left alive until now." He remained in the Ottoman Empire for the rest of the war, becoming in April 1916 the "German Military Plenipotentiary at the Imperial Embassy in Constantinople." Despite the title, he was junior to many of the German officers in the Ottoman Empire serving as advisors to and commanders of Ottoman military formations. In 1919, Lossow, now a major general, was part of the transitional force which would become the Reichswehr, the 100,000-man army permitted to Germany under the Treaty of Versailles.
From 1920 to 1923, he was the commander of the infantry school.
On 1 January 1923 he became the commander in Wehrkreis VII, the Reichswehr military region which covered Bavaria. He held this assignment through the attempted Beer Hall Putsch until his replacement in March 1924.
Generalmajor von Lossow became briefly prominent in German history as being, with Gustav Ritter von Kahr, Minister President of Bavaria and Colonel Hans Ritter von Seisser, head of the Bavarian State Police, part of the triumvirate who at that time exercised political control in Bavaria. The political situation in Germany was one of turmoil and political violence. The Bavarian Government under Ritter von Kahr tended to take a line independent of that of the national government of the Weimar Republic in Berlin. When ordered to arrest three of the leaders of some armed groups then currently operating in Bavaria, the triumvirate refused. General von Lossow was ordered by the Commander-in-Chief of the army, General Hans von Seeckt, to arrest the three men and to suppress the daily newspaper of the Nazi Party, the Völkischer Beobachter. This he hesitated to do, and was sacked from his command by General von Seeckt and replaced by General Friedrich Freiherr Kress von Kressenstein. However, Ritter von Kahr defied Seeckt and announced that von Lossow would retain the command. In 1923, many right-wing groups wanted to emulate Mussolini's "March on Rome" by a "March on Berlin". Among these were the wartime General Erich Ludendorff and also the Nazi group, led by Adolf Hitler. Hitler decided to try to seize power in what was later known as the "Hitler Putsch" or Beer Hall Putsch. Hitler and Ludendorff sought support of the triumvirate. However, Kahr, Seisser and Lossow had their own plan to install a nationalist dictatorship without Hitler. On 8 November 1923, Hitler and the SA stormed a public meeting of 3,000 people which had been organized by Kahr in the Bürgerbräukeller, a large beer hall in Munich. Hitler interrupted Kahr's speech and announced that the national revolution had begun, declaring the formation of a new government with Ludendorff. While waving his gun around, Hitler demanded the support of Kahr, Seisser and Lossow. Lossow, Kahr and Seisser were detained. After Hitler left the Beer Hall to supervise the activities of the putschists, Kahr, Seisser and Lossow were released, ostensibly to fulfill Hitler's orders at their respective offices. Instead, the men fled to join the opposition to Hitler. They went to the barracks of the local infantry regiment, where General Jakob Ritter von Danner, Munich garrison commandant and technically Lossow's deputy, met them. Ritter von Danner, who had been directed independently by General von Seeckt to put down the coup, asked if their statements at the Beer Hall was merely a ruse to escape Nazi custody. The triumvirate agreed, fearing the consequences of their initial cooperation with the putschists, and acted to put down the putsch attempt. Lossow ultimately escaped any disciplinary action for his behavior during the putsch attempt, but never held another command.
