William Fletcher was born on 11 October 1872 in Lemsford, Hertfordshire, the son of the Rev. John Price Alcock Fletcher, of Burbage, Leicestershire, and Mary Ann Darker Banks. He attended Leamington College and like his father and grandfather before him entered Caius College, Cambridge, in 1890, where he took honours in the natural sciences tripos and obtained a scholarship to St. Mary's Hospital, Paddington, from which he qualified in 1896. He held a resident appointment at the Metropolitan Hospital, and then spent several years in general practice in Coventry before entering the Malayan Medical Service in 1903 where he stayed till 1908 as surgeon in charge at the General Hospital in Kuala Lumpur.
He served in the Middle East in the Great War as a captain in the R.A.M.C., and in 1915 he married Mary Beatrice Hillman, by whom he had a son George Bernard Hillman Fletcher and a daughter Penelope Obedience Hillman Fletcher. Mary Beatrice was the daughter of William Hillman the bicycle and car manufacturer, and the sister of Evelyn Hillman who married Percy Rowland Hill (see Section E for both).
William's medical career was a highly significant one in the realm of tropical medicine, making breakthrough discoveries in, for example, tropical typhus, malaria and dysentery. He became director of the Institute for Tropical Medicine in 1926 and retired the next year. He was vice-president from 1933 to 1935 of the Royal Society of Tropical Medicine and Hygiene, and held many other important posts in the field.
Although he lived in Kensington, London, he died in Bedford on 18 September 1938 and is buried at Stoneleigh, presumably because of his wife's connections here. His daughter Penelope was married at Stoneleigh on 4 October 1945 to Major John Richard Grogan, MC and Bar, of the Royal Garhwal Rifles. They later lived in Egypt where he was an army officer. His widow Mary Beatrice died in Kensington in September 1959.