As a clinical neurologist, Cassirer specialized on the anatomy of the central nervous system, and made contributions in his research of multiple sclerosis, encephalitis and poliomyelitis.

Among his written works was a new edition (1923) of Oppenheim's Lehrbuch der Nervenkrankheiten für Ärzte und Studierende.

[2] In 1921, Dr. Cassirer was asked to give testimony in regards to the mental condition of Soghomon Tehlirian, a man accused of murdering Talaat Pasha.

Cassirer maintained that Tehlirian was not sane when he carried out the crime due to a psychotic state caused as a result of his family being victims of a war-time massacre.

Cassirer's portrait was painted by renowned artist Max Liebermann in 1918, and later presented to the Tate Gallery in London.