Born in Neath in 1866, he studied medicine at St Bartholomew's Hospital in London. After qualifying as a Fellow of the Royal College of Surgeons (FRCS) at Edinburgh, he obtained the post of surgeon in the Ear, Nose and Throat Department at the Prince of Wales General Hospital in Tottenham.
He also practised privately at 18 Upper Wimpole Street and was employed as a consultant by the Royal Opera House, Covent Garden. The singers he attended included Adelina Patti, Nellie Melba, and Kirsten Flagstad.
Appointed Laryngologist to His Majesty's Household in 1910, Rees also treated King George V of the United Kingdom as well as several other members of the royal family. He was knighted in 1916 and created Knight Commander of the Royal Victorian Order (KCVO) in 1923 and Knight Grand Cross (GCVO) in 1934 for his services to the royal household. In 1931 the University of Wales awarded him the honorary degree of Doctor of Science.
Sir John retired to Broadstairs in Kent, where he died in 1952.