John MacIntyre was born in High Street, Glasglow, on 2 October 1857. He was originally trained as an electrical engineer and worked as an electrician's apprentice until he enrolled at the University of Glasgow in 1878. It was there that he switched to the field of medicine, graduating on this matter in 1882.
He began his professional career as a surgeon in London, Paris and Vienna, returning to Glasgow shortly thereafter to take up a post as throat surgeon at Anderson's College Clinic.
MacIntyre is known for applying his knowledge of electrical engineering to medicine. In 1885 he became a Consultant Medical Electrician at the Glasgow Royal Infirmary, where he set up a "department for the application of medical electricity" in 1887. In 1893 he served as president of the British Society of Laryngology and in 1894, as part of his interest in the larynx, he created the first self-illuminated endoscope. In 1895 he was elected a Fellow of the Royal Society of Edinburgh.
In late 1895, X-rays were discovered by Wilhelm Roentgen. On February 5, 1896, MacIntyre was asked by James Thomson Bottomley (British physicist noted for his studies of thermal radiation) to demonstrate an X-ray machine created by your uncle William Thomson (British physicist and mathematician especially known for having developed the Kelvin temperature scale). The demonstration took place at the Glasgow Royal Infirmary and was the first use of X-rays in Scotland. In the next 12 months, all Scottish hospitals put up X-ray machines.
MacIntyre went further and in 1896 he created the world's first department of radiology at the Glasgow Royal Infirmary, where X-ray photographs were used in the diagnosis and treatment of patients. There the first images of kidney stones and various internal parts of the body were taken. For this pioneering work he received many awards and honors.
In 1897 he moved to London and founded the London Roentgen Society, serving as its first president. He was also President of the West of Scotland Branch of the British Medical Association, Corresponding Member of the American and French Laryngological Associations, Fellow of the Royal Society of Edinburgh and the Royal Microscopical Society, among other positions.