He was the eldest of nine children of Stephen Mackenzie (1803-1851), a general practitioner and surgeon, and Margaret Frances (died 1877), daughter of wine merchant Adam Harvey, of Lewes, East Sussex.

After going through the medical course at the London Hospital and becoming a member to the Royal College of Surgeons in 1858, he studied abroad in Paris, Vienna and Budapest where he learned the use of the newly invented laryngoscope under Johann Czermak.

So great was his reputation that in May 1887, when the crown prince of Germany (afterwards the Emperor Frederick III) was attacked by the affection of the throat of which he ultimately died, Morell Mackenzie was specially summoned to attend him.

The question was one not only of personal but of political importance, since it was unsure whether any one suffering from an incapacitating disease like cancer could, according to the family law of the Hohenzollerns, occupy the German throne, and there was talk of a renunciation of the succession by the crown prince.

After this sensational episode in his career, the remainder of Sir Morell Mackenzie's life was uneventful, and he died somewhat suddenly in London, on 3 February 1892 and was buried in the churchyard at Wargrave in Berkshire where he had a house in the country.