After the general command was dissolved, he was military commissioner for the government of Lower Bavaria in Landshut between June and October 1919.

While in this post, he acted as an intermediary between the army units and the rebels during Adolf Hitler's failed Beer Hall Putsch on 9 November 1923, for which he was later awarded the Blood Order.

[4] On 1 February 1924, he was promoted to Oberstleutnant and later was transferred to Öls (today, Oleśnica) as a Führergehilfenausbildung instructor for Reiter Regiment 8, from July 1925 until October 1926.

[5] On 1 April 1928, Haselmayr joined the Nazi Party (membership number 85,034) at the Munich Ortsgruppe (local group).

From 1929 to 1933, he was active as a freelance writer and was an employee on the staff of the largest Nazi newspaper, the Völkischer Beobachter, writing on military matters.

In April 1929, he used this forum to praise General Erich Ludendorff, a prominent Nazi supporter, on the occasion of his sixty-fourth birthday.

He praised Ludendorff's strategic genius and alleged that much of the credit that was due to him went instead to Generalfeldmarschall Paul von Hindenburg, then the President of Germany.

[7] Haselmayr was named the representative for military policy questions in the Nazi Party Reichsleitung (national leadership) in 1930.

He was concurrently Epp's deputy leader in the Military Policy Office of the Supreme SA Leadership (OSAF) from September 1932 to October 1935.

He was designated as a Führer for special assignments in OSAF on 27 January 1934, a position he would hold until the fall of the Nazi regime.

In September 1935, he became the founder and leader of the Arbeitsgemeinschaft für Wehrgeistige Forschung (Working Group for Military Spirit Research).

In October of the same year, he was appointed to the council of experts of the Reichsinstitut für Geschichte des neuen Deutschlands [de] (Reich Institute for the History of the New Germany).

Transferred to the eastern front in December 1941, he served as the Oberfeldkommandant (senior field commandant) in the security region Volhynia-Podolia.