He was born in Bunzlau, Lower Silesia; (today known as Bolesławiec, Poland) into a Jewish family.

[1] He studied medicine in Berlin and in 1871 received his habilitation for psychiatry.

[2] Mendel was an advocate in regards to the unification of psychiatry and neurology as complementary disciplines.

Among his better-known students and assistants were Max Bielschowsky (1869–1940), Edward Flatau (1869–1932), Lazar Minor (1855–1942) and Louis Jacobsohn-Lask (1863–1940) Mendel is remembered for the introduction of duboisine, an extract from the Australian plant Dubosia myoporoides, as a treatment for Parkinson's disease.

Also, he conducted important studies of epilepsy and progressive paralysis.