In 1887, he received his doctorate of philosophy, afterwards working as an assistant in the laboratory of Wilhelm Wundt (1832-1920) at Leipzig.

In 1904, he was a co-founder of the Gesellschaft für experimentelle Psychologie (Society for experimental Psychology).

[3] In 1904, Sommer, together with his colleagues Georg Elias Müller, Hermann Ebbinghaus, Oswald Külpe, Ernst Meumann and Friedrich Schumann, convened the first "Congress for Experimental Psychology" in Giessen, at which the Society for Experimental Psychology was founded.

Sommer died in 1937 of pneumonia contracted during a six-hour winter hike.

On the occasion of the 100th anniversary of the Giessen Psychiatric Clinic, the Robert Sommer Award Symposium was established, at which international scientists have since presented and discussed research results on schizophrenia, and the Robert Sommer Medal has also been awarded for special services in schizophrenia research.