Sidney Garside studied at Victoria University, Manchester, where he obtained the degrees Bachelor of Science (BSc) in 1910 and Master of Science (MSc) in botany in 1911. After a brief period as lecturer in botany at the Royal Technical College, Salford (near Manchester) he came to South Africa to take up an appointment from January 1912 as lecturer in botany at Victoria College, Stellenbosch (later the University of Stellenbosch), where he assisted Miss A.V. Duthie*. He built up his own herbarium of plants, collected on bicycle trips, and also collected with H.W. Rudolf Marloth*.
Garside returned to England in December 1920 and became a lecturer in botany at Bedford College (a women's college of the University of London in Bedford, some 80 km north of London). His research, which focussed on the floral anatomy and pollen of the Proteaceae, led him to visit South Africa six times between 1929 and 1938, the first time with the British Association for the Advancement of Science. In 1938 he retired, sold a collection of 2500 plants to the herbarium at the Royal Botanic Gardens at Kew, near London, and in November that year returned to South Africa and settled in Cape Town. There he continued his research as a volunteer worker in the Bolus Herbarium and an honorary reader in systematic botany of lower plants at the University of Cape Town. At the Bolus Herbarium he worked mainly on the bryophytes and lichens and expanded its collections of these groups.
Garside's research and publications dealt with a variety of topics, for example, "Pollen presentation in Cryptostemma calendulaceum R.Br." (Fam. Compositae; Annals of the Bolus Herbarium, 1918); "A revision of the characters of Neodrega" (Fam. Liliaceae; Kew Bulletin, 1935); "The South African species of Spiloxene Salisb." (Fam. Amaryllidaceae; Journal of Botany, 1936), "Studies in South African Ricciaceae" (Liverworts; by A.V. Duthie and Garside; Transactions of the Royal Society of South Africa, 1936, 1939, 1958); "Baron Jacquin and the Schoenbrun Gardens" (in Vienna, Austria; Journal of South African Botany, 1942); "The developmental morphology of the pollen of Proteaceae" (ibid, 1946); and "Sphagnum in South Africa" (moss; ibid, 1949). He also contributed sections on the genera Spiloxene, Pauridia and Empodium (Fam. Amaryllidaceae) to the Flora of the Cape Peninsula by R.S. Adamson and T.M. Salter (1950). However, although he studied the South African lichens for many years, his notes remained unpublished.
Garside was elected a Fellow of the Linnean Society of London in 1922 and served on its council from 1935 to 1937. He married Minnie Collins, born Hofmeyr, in 1939.