Husband of D. H. Pienaar, of Pretoria. He joined the artillery branch of the Natal Police (NP) in 1911, and transferred to the Union Defence Forces (UDF) when they took over the NP in 1913. In World War I, he served as an artilleryman with the South African Overseas Expeditionary Force in German East Africa and Palestine. Between the two world wars, he held various staff and command posts. In World War II, he commanded the 1st South African Infantry Brigade in the East Africa Campaign (battles of El Wak, The Juba, Combolcia, and Amba Alagi) in 1940-1941, and the North Africa Campaign (battles of Sidi Rezegh and Gazala) in 1941-1942. At Gazala, he was promoted to command the 1st South African Division, which he led in the battle of Gazala, the retreat to Egypt, the defence of El Alamein, and the final battle of El Alamein. He was killed in an air crash in Kenya, on his way back to South Africa. Maj Gen Pienaar was arguably one of South Africa's most charismatic and popular military commanders. An infantry regiment, and a suburb of his home town, Bloemfontein, were later named after him.
DSO LG 14 February 1941; Bar to DSO LG Sup 9 September 1942 - "For gallant and distinguished services in the Middle East during the period November, 1941, to April, 1942"; MID LG Sup 30 December 1941; MID LG Sup 24 June 1943 pg. 2867