Francois Johannes Jansen, son of Ernst Johannes Jansen and his wife Cecilia Anna, born Swanepoel, passed the matriculation examination of the University of the Cape of Good Hope in 1887. He was employed as a clerk in the magistrate's office in Fraserburg from February 1889. Subsequently he served in a similar position at Beaconsfield (January 1890); as examiner of accounts in the Colonial Secretary's Office, Cape Town (October 1892); as magistrate's clerk at Barkly West (November 1894), Aliwal North (April 1895), and Woodhouse (May 1895); and as chief clerk and paymaster at Kimberley (November 1897). After brief appointments during the Anglo-Boer War (1899-1902) he became inspector of war claims in the prime minister's office (April 1903); registrar of the Eastern Districts Court in Grahamstown (April 1904); civil commissioner and magistrate at Murraysburg (October 1907); and resident magistrate at Victoria West (December 1910). Some years after the formation of the Union of South Africa in 1910 he was appointed as chief clerk in the office of the minister of justice in Pretoria (1917); as chairman of the Rent Board for the district of Uitenhage (1923); and as a member of the Licensing Board for the Pretoria District (1929).
Jansen collected prehistoric stone artefacts, mainly in the vicinity of Victoria West, but also at Vosburg, Meltonwold, and Loxton in the Cape Province. He presented many stone artefacts to the Albany Museum in 1913. In 1918 he sent a collection of stone implements from Victoria West to the British Museum (Natural History), which were briefly described by Reginald Smith in "Recent finds of the stone age in South Africa" (Man, 1919). Later Jansen himself published a single archaeological paper, "A new type of stone implement from Victoria West" (South African Journal of Science, 1926, Vol. 23, pp. 818-825). In this seminal paper on the Victoria West culture he argued that the artefacts were finished tools, though later work showed them to be prepared cores from most of which a single large flake had been struck. Jansen also collaborated with the archaeologists Christoph H T D Heese and Astley J H Goodwin in further investigations of the Victoria West culture, the result of which were published by Goodwin and C van Riet Lowe in The stone age cultures of South Africa (1929).
Jansen was married to Anna Catharina, born Viljoen, with whom he had a son.