Peter D. Strachan qualified as Master of Arts (MA), and in 1898 as Bachelor of Medicine (MB) and Bachelor of Surgery (ChB). He was licensed to practice in the Orange River Colony (now the Free State) on 25 August 1902. During that year he worked at the Springfontein refugee camp, which was established during the Anglo-Boer War (1899-1902), but in September 1902 resigned to accept an appointment as district surgeon of Phillipolis. He still held this position in 1912. However, for some time he was medical officer at Serowe, Bechuanaland (now Botswana). In 1911 he was awarded the degree Doctor of Medicine (MD) by the University of Glasgow and in 1916 resided at Taynuilt, Argyllshire, Scotland. Later he returned to southern Africa and by 1924 was superintendent of the Botsabelo (or Botsabelu, Botshabelo) leper asylum near Maseru, Basutoland (now Lesotho). In 1928 he was offered an appointment as bacteriologist in the Human Trypanosomiasis Research Institute at Entebbe, Uganda, but it is not clear whether he accepted.
Strachan made some investigations into brucellosis in South Africa - a disease caused by bacteria of the genus Brucella and also known as Malta fever or undulant fever. Two papers resulting from his work were published in the South African Medical Record: "The question of the presence in South Africa of Malta or undulant fever" (1904) and "Undulant fever in South Africa" (1906). In the latter paper he discussed South African cases of the disease during the eighteen-nineties.
Strachan was an early member of the Astronomical Society of Southern Africa, formed in 1922. He was still a member, and residing at Botsabelo, in 1938.