Cornelis Pijper, medical practitioner and student of rock art, was the elder brother of Adrianus Pijper* and the son of Fredrik Pijper, professor of church history at the University of Leiden, and his wife Maria Petronella, born van Wijk. He qualified as a licentiate in medicine and surgery of the Society of Apothecaries (London) in 1910. That same year he came to South Africa, was registered to practise medicine in the Transvaal, and settled in Bethal. During 1915-1916 he spent a few months in Pietersburg (now Polokwane), but by 1917 he had moved to Lydenburg, and by 1926 to Johannesburg. In March 1919 he returned temporarily to Holland and that same year was awarded the degree Doctor of Medicine (MD) by the University of Leiden, with a thesis entitled De volksgeneeskunst in Transvaal (Folk medicine in the Transvaal).
During 1915-1916 Pijper published five articles in the Nederlands Tijdschrift voor Geneeskunde, on his experiences as a medical practitioner among the indigenous population of the Transvaal. Later medical papers by him included the following: 'Medical folk-lore of the Abantu in the Lijdenburg district, Transvaal' (1921) and 'The account book of Jan Haszing' (1922) in the Transactions of the Royal Society of South Africa; 'Hits and misses in the diagnosis of skin eruptions' (1921/2) and 'Notes and reflections on skin diseases' (1922/3) in the Medical Journal of South Africa; 'The evaluation of immunisation against diphtheria' (South African Journal of Medical Sciences, 1937); and 'Expectation of life and expectation of cancer in the Union of South Africa' (South African Medical Journal, 1943).
Pijper's interest in rock art led to two published papers: 'Some engraved stones of the Lydenburg district and north-east Transvaal: The occurrence of cup-and-ring markings in South Africa' (South African Journal of Science, 1918), and 'A prehistoric rock-sculpture from the north-eastern Transvaal' (Transactions of the Royal Society of South Africa, 1921).
Pijper became a member of the South African Association for the Advancement of Science in 1917. He was married, before 1915, to Suzanna Hermine Pijper.