Douglas Flemmer Gilfillan, was the eldest child of Edward Stockenstrom Lodewikus Gilfillan, solicitor at Cradock, and his wife Charlotte Marie Louise Flemmer. After completing his schooling in Cradock he matriculated through the University of the Cape of Good Hope in 1881. Thereafter he articled in Cape Town, passed the examination for the Certificate of Proficiency in Law and Jurisprudence of the University of the Cape of Good Hope in 1886, and that same year was admitted to practice as solicitor and notary to the Supreme Court of the Cape Colony. He practiced in Cradock until 1888, then moved to Barberton, in the South African Republic (Transvaal), and in 1889 started practicing there as a solicitor, notary, conveyancer and sworn translator. He joined the Barberton Scientific and Literary Society in August 1889 and met the plant collector E.E. Galpin*, who became a life-long friend. In 1890 he married Sophia Maria de Jongh of Cape Town and they eventually had four children. Galpin later married Sophia's older sister, so that he and Gilfillan became brothers-in-law. Gilfillan left Barberton in 1892 for Johannesburg and later became a partner in the firm Hoyle & Gilfillan. By 1896 he was a member of the Reform Committee, an organisation of Johannesburg immigrant leaders attempting to obtain an increased political role for immigrants in the South African Republic, and was imprisoned with many others following the Jameson Raid. During the Anglo-Boer War (1899-1902) he saw active duty as a captain in the Imperial Light Horse and after the British occupation was appointed a member of the special criminal court for Johannesburg and acting magistrate for Boksburg and Germiston. In 1902 he became a partner in the legal firm Bowman & Gilfillan (later Bowman, Gilfillan & Blacklock), where he remained for the rest of his career.
Gilfillan was interested in nature from an early age. Galpin kindled his enthusiasm for botanical collecting and he collected over 500 specimens: around Johannesburg from November 1898 to January 1899; on Conway Farm, Middelburg (Cape) in July and September 1899; and in the Heidelberg and Witbank districts in November 1905. His specimens were sent to Galpin, who incorporated them in his herbarium under Gilfillan's name. Galpin's herbarium later formed the nucleus of the government herbarium in Pretoria. Gilfillan was commemorated in the species names Canthium gilfillanii and Zygophyllum gilfillanii by N.E. Brown*, and Euryops gilfillanii by H. Bolus*.
Gilfillan's other major interest in the field of natural history was ornithology. He became a member of the South African Ornithologists' Union in 1907, and the next year contributed "Notes on some game birds of South Africa" to its Journal (Vol. 4(1), pp. 19-36). There appear to have been other ornithological papers by him as well. Around 1918 he became a member of the South African Biological Society. He was furthermore one of the founders of the Wild Life Protection Society of the Transvaal and for many years its president. In 1938 he became a member of the National Parks Board of Trustees (established in 1927).