Following the outbreak of the First World War, he served as a platoon leader with the rank of Stabsfeldwebel, and saw action on the eastern front until 1 August 1915, when he was captured by forces of the Imperial Russian Army.

However, by March 1919 he had switched sides to join Walter Riehl's Deutsche Nationalsozialistische Arbeiterpartei and he became a convinced activist in the party after hearing Adolf Hitler speak at Krems in 1920.

[8] Leopold was appointed the Landesleiter of the banned Austrian Nazi Party on 29 January 1935 and the leader of all SA troops in Austria in June.

[9] Over a year later, on 11 July 1936, Hitler concluded an agreement with Chancellor Kurt Schuschnigg, recognising Austria's sovereignty and agreeing that Austrian National Socialism was a "domestic matter" over which Germany would exert no direct or indirect influence.

[12] He then sought to work more closely with other nationalist groups in order to come closer to his ideal of a Nazi Austria and, in this endeavour, won the backing of cabinet ministers Edmund Glaise-Horstenau and Odo Neustädter-Stürmer [de].

[14] Indeed, Leopold came out of the negotiations badly, as not only were his plans to reconstitute the Nazi Party as part of the Fatherland's Front made public, but he also failed to secure an offer of a cabinet post as he had hoped.

[15] His failures in the negotiations and his pro-independence stance meant that Hitler did not trust Leopold, who he felt might use the SA to try to seize power on his own, perhaps by force.

[18] Hitler finally dismissed Leopold as Landesleiter on 21 February 1938, on the pretext that he wanted Austrian Nazis to follow legality.

[16] Hitler, who had been due to hold talks with Schuschnigg, was especially annoyed that Leopold had launched a bombing campaign in the run-up to the meeting, and so acted quickly to remove him.

[15] Following the Anschluss, Leopold was able to continue his career within the Nazi Party in Germany, despite his earlier support for an independent Austria.

With the rank of Oberstleutnant, he commanded of a reserve battalion of Austrian troops in the Battle of France and in the invasion of the Soviet Union.

[20] In October 1945, Leopold's widow, Sidonie née Saxeneder (16 August 1894 – 19 July 1980) was tried by the Vienna People's Court for her Nazi Party membership.