He was born in Bond Head, Canada West (now Ontario) and trained at McGill University in Montreal, where he studied the field of medicine. He obtained his MD at McGill University. Later in his life he went south to Philadelphia and was appointed Chair of Clinical Medicine in the University of Pennsylvania. In 1889 he became the first Professor of Medicine in Johns Hopkins University. He later moved to England in 1905 and pursued his career as the Regius Chair of Medicine at Oxford, which he held until his death. Osler was named a baronet in 1911 due to his great contributions to the field of medicine.
Osler was a great collector of books relevant to the History of Medicine. After his death, his collection formed the nucleus of McGill University's Osler Library of the History of Medicine, which opened in 1929.
Throughout his life Osler was a great admirer of the 17th century physician and philosopher Sir Thomas Browne.