In 1895 he was appointed visiting physician to the hospitals of Paris, and in 1904 became an instructor in the faculty of medicine.
In 1905 he became a physician to the Hôpital Cochin, and was in charge of the medical clinics at the same institution.
Widal was the author of a remarkable series of essays on infectious diseases, erysipelas, diseases of the heart, liver, nervous system, etc., besides being a prolific contributor to various medical journals and encyclopedias.
His name is associated with the Widal test, a diagnostic test for typhoid fever, and with hematologist Georges Hayem (1841–1933), he described acquired haemolytic anaemia, a disease that was historically referred to as "Hayem–Widal syndrome".