P.J. Pienaar was appointed as a clerk in the Mycology Section of the Department of Agriculture of the Transvaal Colony in August 1909, to work for the mycologist I.B. Pole Evans*. After the formation of the Union of South Africa in 1910 he continued his work from April 1912 as a third grade clerical assistant under Pole Evans in the Division of Plant Pathology and Mycology of the Department of Agriculture. In 1916/7 he became a foundation member of the South African Biological Society, but he was no longer a member by 1920.
Pienaar became interested in fungi and made a comprehensive collection on the farm Garsfontein, near Pretoria, where he lived. In 1916, during the citrus canker campaign, he was sent on some inspection trips over a wide area and from all these brought back interesting specimens of both fungi and flowering plants. The species Aloe pienaarii was named after him by Pole Evans and the fungus Ravenelia pienaarii by E.M. Doidge*.