On commencing his medical studies Mackenzie determined to take his degree at the University of London, combining with his hospital work the preparation for matriculation. Having become a member of the College of Surgeons in 1858, he subsequently took the M.B. degree with high honours in three subjects. At the London Hospital he obtained the senior gold medal for surgery, and the gold medal for zeal, talent, and humanity to the patients, awarded by the governors. On leaving the hospital he went to Paris, where he studied for a year under Trousseau, Nélaton, Ricord, and others. He spent another year in Vienna, where he studied pathology under Rokitansky, chest diseases under Skoda, skin affections under Hebra, and diseases of the eye under Arlt and Jäger. During his stay at Vienna Mackenzie made an expedition to Pesth in order to become acquainted with the laryngoscope, an instrument invented by Manuel Garcia, which Czermak was then beginning to use. A friendship sprang up between these two men which only terminated with Czermak’s lamented death. Czermak was very desirous that Mackenzie should translate some of his papers and publish them in the English medical journals, but he had determined to study for a few months in Italy, and before he returned home Czermak had himself come over to London and introduced the laryngoscope into England. On arriving in London Mackenzie was at once appointed Resident Medical Officer at the London Hospital, and shortly afterwards Registrar to that institution. He now began to make daily studies with the laryngoscope, and soon published cases in the medical journals which had been treated by its aid. In 1862 he completed the M.D. degree at London University. In 1863 the Jacksonian prize for an essay on the Diseases of the Larynx was awarded to Mackenzie by the Royal College of Surgeons, and on the urgent advice of many of his medical friends, especially that of the late Dr. Herbert Davies, he determined to make throat diseases a specialty, and having established himself in practice in the West End, he was largely instrumental in founding the Throat Hospital in King Street, Golden Square, in the same year. In 1866 Dr. Mackenzie was appointed Assistant-Physician to the London Hospital, and his colleagues subsequently offered to recommend to the committee of that institution that a department for throat diseases should be established under his supervision. This however he declined, on the ground that he wished to treat diseases of every kind whilst attached to the London Hospital. He, however, gave a course of lectures on Throat Diseases at the London Hospital Medical College, whilst he also lectured on Physiology for three years. Dr. Mackenzie was afterwards obliged, owing to his increasing practice, to resign his connection with the London Hospital.