Cornelis J. Swierstra, son of Johannes N. Swierstra and his wife, Jankje Meyer, studied entomology at the University of Amsterdam and gained some museum experience in the Netherlands before he came to the South African Republic (Transvaal) early in 1897 and settled in Pretoria. Around May that year he was appointed as entomological assistant at the Staatsmuseum (later the Transvaal Museum), under its first full-time director, Dr J.W.B. Gunning*. He was also put in charge of the museum's ethnographic collection. In 1899 he became a citizen of the South African Republic and at the start of the Anglo-Boer War in October that year was called up for military duty in the Hollander Corps. After the British occupation of Pretoria, at Gunning's request, he was re-appointed to his post at the museum in September 1900, with responsibilitiy for the entomological, ethnographical and historical collections. He spent his entire career at the museum, progressing to the rank of first assistant (assistant director) in 1906 and serving as director from 1922 to his retirement in 1946. He was married to Niesje Swierstra*, born Kwak, with whom he had one son and one daughter. He was survived by his second wife, Antonia J. Franken
During his early years at the museum Swierstra collected many Lepidoptera, but also other insects, spiders, scorpions and other zoological specimens. For example, during 1905 he collected Hymenoptera, Coleoptera and Diptera along the Plat River, north of Pretoria; then went on a collecting trip to the Zoutpansberg district with the museum's taxidermist, F.O. Noome*, collecting insects, birds, fishes and ethnographic specimens from October to December that year; and the next year went on excursions to the Magalies River and the Pienaars River with the museum's botanical assistant, Miss R. Leendertz*, where he collected insects and fishes. Many of the species he brought back were new to the museum's collections. In 1914 he led the first Tsetse Fly Research Expedition to Zululand, where he was assisted by R.E. Symons*.
In 1909 Swierstra published his "Check list of Lepidoptera-Rhophalocera [butterflies] of the Transvaal, with notes on some of the species", in the Annals of the Transvaal Museum (Vol. 1(4), pp. 235-299). Other contributions by him published in the Annals dealt with some rock engravings found in the Transvaal (1908), descriptions of three new butterfly species (1909), and a collection of butterflies made in Mozambique by the director of the museum, Dr H.G. Breijer (1915).
In January 1908 Swierstra became a foundation member of the Transvaal Biological Society and in 1916 a foundation member of its successor, the South African biological Society. For a short period around 1906 and again from 1915 he was a member of the South African Association for the Advancement of Science. He became a member of the Royal Society of South Africa in 1908. In recognition of his work on insects he was elected a Fellow of the Royal Entomological Society. He became a foundation member of the Entomological Society of South Africa in 1937. In April 1936 he convened and chaired a meeting in Kimberley at which the South African Museums Association (SAMA) was formed and was elected its president for the first year.