He published over 300 scientific papers on neurology topics including cerebral localization, electrotherapeutics, aphasias and the effects of tumors in the central nervous system.

In 1900, he first described a case of ascending paralysis, a rare motor neuron condition that has become known as Mills' syndrome.

He was later commissioned 1st corporal in the 33rd regiment and fought against the Army of Northern Virginia as they retreated from the Battle of Gettysburg.

[2] Mills worked as a general practitioner for several years before turning his focus to nervous and mental diseases.

The only other neurologist at the time was Silas Weir Mitchell whose work on Civil War injuries created the framework for the specialty of neurology.

[2] Mills was invited to participate in the high profile autopsy of Charles J. Guiteau, who was hanged for the 1882 assassination of President James A. Garfield.

In 1917, he trained the U.S. Medical Reserve Corps on the treatment of nervous disorders arising from World War I battles.

[1] His research focused on cerebral localization and described how certain sections of the brain were dedicated to motor and sensory activities.

He helped pioneer neurosurgery and studied diseases such as athetosis, hysteria, insanity, neuritis, myotonia and polio.