Leonard Guscott Haydon, son of George Haydon and his wife Ann, born Guscott, qualified as Bachelor of Medicine (MB) and Master of Surgery (CM) at the University of Aberdeen, Scotland, in 1896. The next year he acquired the Diploma in Public Health at the same university. He then worked for the British Government and was sent to fight an outbreak of plague in Bombay (now Mumbai), India. In May 1898 he went to Victoria, Australia, where he did locum work for his brother, Dr Gerald Haydon.
By 1901 Haydon had moved to Cape Town and was engaged by the government of the Cape Colony to combat plague in the city. In November that year, during the Anglo-Boer War (1899-1902), he accepted a post at the Heilbron refugee camp in the newly created Orange River Colony. The next year, in response to an outbreak of plague in Mozambique, he was appointed medical inspection officer at Ressano Garcia, Mozambique, opposite Komatipoort.
In December 1902 Haydon joined the Health Department of Natal and did some research at the Allerton Veterinary Laboratory in Pietermaritzburg. He applied for registration as a medical practitioner in the colony in 1903. In October 1904 he was appointed assistant port health officer for Natal and that same year carried out some experiments on the comparative effectiveness of disinfectants at the Point Laboratory in Durban. He was also involved in preparations against a possible plague epidemic this year. In 1906 he was appointed (in addition) medical officer to the temporary goal at Jacobs, Durban. He became involved in studying mosquitoes as carriers of malaria and, as co-author with Dr Ernest Hill*, published "The epidemic of malarial fever in Natal" (Journal of Hygiene, 1905) and"A contribution to the study of the characteristics of larvae of species of Anophelina in South Africa" (Annals of the Natal Museum, 1907, Vol. 1(2), pp. 111-158).
After the formation of the Union of South Africa in 1910 Dr Haydon was appointed assistant medical officer of health for the Union in September 1911, stationed in Durban. During World War I (1914-1918) he served with the South African forces in German South West Africa (now Namibia), attained the rank of Lieutenant-Colonel and was honoured as a Companion of the Distinguished Service Order (DSO). In 1920 he was appointed in the administrative division of the Department of Public Health and transferred to Pretoria. He was still employed by the department by 1926. After his retirement he moved to Australia, residing in Melbourne and in Glenthompson, Victoria. He was survived by his wife, Betty Haydon, but had no children.