From 1886 to 1888 he devoted himself to public demonstrations of the researches of the faculty of pathological anatomy, and during the 2 years following was in charge of a course in bacteriology in the laboratory of Professor Victor André Cornil. 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, which is a diagnostic test for typhoid fever, and with hematologist Georges Hayem he described acquired haemolytic anaemia, which was historically referred to as the "Hayem-Widal syndrome".