Pieter Joseph Dormehl, son of Siebert Frederick Dormehl and his wife Gesina Maria, born Fink, started his career as a clerk in the Colonial Secretary's Department of the Cape Colony in July 1891. The next year he was transferred to the office of D.E. Hutchins*, conservator of forests of the Western Conservancy. In 1899 he was appointed as a district forest officer (Western Conservancy), but that same year the government of the Cape Colony sent him to be trained as a forester at the Cooper's Hill School of Forestry in England. Upon his return in 1902 he was appointed as district forest officer at Knysna. He became a justice of the peace at George in 1907.
On 1 March 1909 Dormehl was promoted to assistant conservator of forests in the Western Conservancy, stationed in Cape Town, and that same year was appointed as justice of the peace in that city. In 1913 he succeeded C.E. Legat* as conservator of forests of the Orange Free State (now the Free State), a post he held until his retirement at the age of 50 in 1923.
Dormehl was a prominent rugby player in a forward position. In 1896 he played in two matches for South Africa against the visiting British team. After his retirement he settled in the Knysna area. He obtained an irrigation loan in 1940 and sold a portion of the property Redlands to the Department of Forestry. He was married to Winifred Creighton, with whom he had five children.