He received his medical degree (1873) in Montpellier, where in 1881 he became a professor of therapy. In 1886, he attained the chair of clinical medicine, and in 1909 was appointed chair of general pathology.
Grasset was involved in every aspect of internal medicine, but his primary interest concerned diseases of the nervous system. His name is associated with the "Grasset law", 'a condition where a patient with hemiparesis lying on his back can raise either leg separately, but is unable to raise both legs together.':649 This phenomenon is explained in his 1899 treatise, Diagnostic des maladies de la moëlle.
He conducted studies in the field of psychiatry, publishing the book Demi-fous et Demi-responsables (Semi-Insane and the Semi-Responsible) in 1907, and also researched the paranormal, publishing works with titles such as Le spiritisme devant la science (1904) and L’occultisme hier et aujourd'hui (1907).
- Biography of Joseph Grassett ("Who named it?").
- Clarac, François; Boller, François (2009-01-01), Aminoff, Michael J.; Boller, François; Swaab, Dick F. (eds.), "Chapter 40 History of neurology in France", Handbook of Clinical Neurology, History of Neurology, Elsevier, 95, pp. 629–656, retrieved 2020-06-03
- The diagnosis of diseases of the cord (Michigan: George Warr, 1901).
- The semi-insane and the semi-responsible (New York : Funk and Wagnalls, 1907).
- The marvels beyond science : (L'occultisme hier et aujourd'hui; le merveilleux préscientifique) (New York; Funk & Wagnalls company, 1910)