[1] Bank clerk and merchant by occupation, Mahrer played professionally in Czechoslovakia and the United States.
In 1923, he joined DFC Prag before moving to the United States in 1926 to sign with the Brooklyn Wanderers of the American Soccer League.
At that time, he joined the Hakoah All Stars in the Eastern Professional Soccer League.
[3] In 1929, Hakoah moved to the American Soccer League, where Mahrer played until the fall of 1931.
After the German occupation of the Sudetenland, he was imprisoned at the Theresienstadt concentration camp on account of his Jewish ethnicity.