Petrus Johannes Cillie, son of Charl Daniel Cillie and his wife Martha Helena Marais, was born on the family farm Rheebokskloof in the Wellington district and worked on the farm in his young days. In 1883 he bought the farm Welbedacht in Bo-vlei, just north-east of Paarl, concentrating on the production of high quality dried fruit for export. In 1892 he was a valuable witness for the parliamentary Select Committee on Fruit Culture and Fruit Export, and the next year was sent to investigate fruit culture in California, United States, by the Department of Agriculture of the Cape Colony. He sent back some useful technical reports, for example on the artificial ripening of lemons and on the tortoise beetle, Orcus chalybeus, used to control insect pests on citrus trees. These reports were published in the Agricultural Journal of the Cape of Good Hope. After his return Cillie, who came to be known as "Piet Kalifornie", sometimes served as an agricultural extension officer to advise farmers. Reports on some of his visits to farming districts appeared in the Agricultural Journal. He also brought back seeds, plants, agricultural machinery and implements and lectured on fruit culture at Hex River, Goudini, Robertson and Montague during 1894-1895.
In 1895 Cillie compiled a Selected list of fruit trees, recommended for cultivation, giving full information regarding size, use, quality, season, and origin of selected varieties, a 40 page monograph published in Paarl. In 1899, as co-author of C. Mayer*, he participated in writing a pamphlet, Report on failure of hanepoot grafts on American vines, which was distributed by the Cape Department of Agriculture. A few years later he was appointed, with F.T. Bioletti and W. Gowie, as a member of the Commission of Inquiry into the causes of the failure of citrus trees in Cape Colony. Their report, Citrus culture in Cape Colony, was published by the Cape Department of Agriculture in 1904. He became managing director of the Pioneer Dried Fruit Factory in Bo-vlei, and was chairman of the Wellington Fruit-Farmers' Association for 34 years.
Cillie offered to plant and experiment, on his own farm and at his own expense, with fig trees that had been imported by the Department of Agriculture. This served as an example for later cooperative research by farmers. Later he played an important role in developing the settlement at Kakamas. He served on the Labour Colony Commission from 1920, and served as its chairman from 1922 to his death in 1942. In 1974 a variety of apricot developed at the Research Institute for Fruit and Fruit Technology was named the Piet Cillie apricot in his honour. He was married to Maria Susanna Malan and had two sons and a daughter.