Mergenthaler was born in the town of
Waiblingen in
Württemberg, the son of a baker. He attended the local
Volksschule between 1894 and 1898 and then graduated from the
Gymnasium in
Bad Cannstatt in 1902. After studying mathematics and physics at the
Technical University of Stuttgart,
Tübingen University and the
University of Göttingen, he passed the first service examination for secondary school teachers in 1907. He performed mandatory military service as a
one-year volunteer between 1908 and 1909 with the 13th (Hohenzollern) Foot Artillery Regiment, headquartered in
Ulm. He then passed the second state examination in 1911, and was employed as a senior teacher in the grammar and high schools in
Leonberg. He returned to military service during the
First World War as an
artillery battery commander in the 24th Reserve Foot Artillery Regiment, much of that time at the
front. After the end of the war, he left the army with the rank of
Oberleutnant of
reserves. In 1920, Mergenthaler became a
Gymnasium professor in the town of
Schwäbisch Hall. A conservative
German nationalist, with an
antisemitic character, his radicalized war experience and sense of post-war social outrage led him to embrace extreme right-wing politics. He co-founded the local chapter of the
Nazi Party in Schwäbisch Hall in 1922, and became heavily engaged in
propaganda activities as a public speaker. After the Party was banned in 1923 following the failed
Beer Hall Putsch, he joined the
National Socialist Freedom Party (NSFP), a Nazi
front organization. In May 1924, he was elected as a member of the
Völkisch-Social Bloc (VSB) electoral alliance to a seat in the
Württemberg Landtag (state parliament), which he would hold until 1928, and again from 1929 until the
dissolution of that body by the Nazis in October 1933. He also won a seat in the
Reichstag from electoral constituency 31 (Württemberg) but only served there until the next election in November 1924. When the ban on the Nazis expired in 1925, the NSFP and the VSB were dissolved and many of its members rejoined the Nazi Party. Mergenthaler hesitated to do so until 1927 because he thought
Adolf Hitler's dictatorial style was harmful to the cause. In a 1928 struggle for the key position of Party
Gauleiter, Mergenthaler was outflanked by
Wilhelm Murr, which resulted in a long-term rivalry between them with Mergenthaler holding the much less educated Murr in contempt. From 1929 to 1932, as the only Nazi Party deputy in the
Landtag, he aggressively pursued the Party's goals. == Career in Nazi Germany ==