Cornelis Frederik Delfos was the son of Frederik Cornelis Delfos and his wife Maria, born Driesman (or Triesman). After completing his secondary schooling in Amsterdam in 1884 he completed an apprenticeship as fitter and turner at the Koninklijke Fabriek voor Stoom en andere Werken (Royal Factory for Steam and other Works). Subsequently he qualified as an engineer at a technical institute in Leipzig, Germany, in 1888 and became a member of the Nederlandsche Ingenieursvereniging (Netherlands Engineering Society). In 1890 he and his brother Johan J.F. Delfos visited South Africa and decided to establish an engineering firm here. After returning to the Netherlands to recruit workmen they emigrated to South Africa in 1892 and established the firm Delfos Brothers, electrical and general engineers, in Pretoria. The firm provided a wide range of services and was very successful. In September 1895 Cornelis Frederik married Johanna Elizabeth Cornelia van Nikkelen Kuyper in Pretoria. They had two sons and a daughter.
During the Anglo-Boer War (1899-1902) the firm continued to flourish as a result of the role it played in the war effort of the South African Republic. Among other activities its staff repaired artillery and manufactured ammunition. Some time after Pretoria fell to the British forces in 1900 Delfos was detained and deported to the Netherlands. While there, in 1901, he was granted a United States patent for a new or improved machine to crush ore down to a powder by means of rotatable hammers in a steel drum.
A few months after the end of the war, in 1902, Delfos returned to Pretoria and at some time became a British citizen. He started working towards establishing a steel industry in Pretoria by obtaining analyses of the Timeball Hill iron ore deposits and experimenting with the coking of Transvaal coal. In 1917 he obtained a concession from the Pretoria city council for the mining of the local iron ore, and founded the Pretoria Iron Mines, Ltd. His company built a small, experimental blast furnace, designed by Professor G.H. Stanley* of the South African School of Mines and Technology in Johannesburg, which came into operation in June 1918 and produced a few thousand tons of pig iron during the next three years. Delfos continued his efforts to find capital and political support for establishing a steel industry until an act to establish the South African Iron and Steel Industrial Corporation (Iscor) was passed in March 1928. He played a major role in this development as an adviser to the government, but declined an offer to become chairman of the new corporation, considering the appointment of a younger person more appropriate. As a result his vital contributions gained him little recognition and his long struggle furthermore impoverished him. He was appointed as a government director of Iscor in 1928 and his appointment renewed for a further period of four years in 1932. However, early in 1933 he was paralysed by a stroke and died later that year. A railway station near the Iscor works in Pretoria was named Cordelfos in his honour.