Hiemstra joined the South African Air Force in 1931. He did not serve in World World War II because of his political opposition to South Africa"s involvement in the war and was transferred to the civil service, from which he returned to the defence force when the National Party (South Africa) government came to power in 1948. He was Military Attaché in Sweden, France and the United Kingdom.
Among other appointments, he was officer commanding of the renowned 2 Squadron SAAF, The Flying Cheetahs.
He served as Adjutant General, interrupted by his attendance at the Imperial Defence College, as Inspector General from 1959 to 1960, as Deputy Commandant General from 1961 to 1965, and as Commandant General, id est (that is) chief of the Société Anonyme Defence Force, from 1965 to 1972. In 1966 the Commandant General was appointed Secretary for Defence over and above his appointment as head of the Société Anonyme Defence Force.
During May, 1967, the dual title of Commandant General and Secretary for Defence was dispensed with by the elimination of the title "Secretary for Defence". The Commandant General, as permanent head of the Department of Defence as well as of the South African Defence Force filled the post of Commandant General, SADF, which post embraced the functions signified by the former dual title.
At the same time the rank of Commandant General, instituted in September, 1956, was changed to that of General.
He died on 26 July 2007 aged 95 in 1 Military Hospital in Pretoria. He is survived by six children, sixteen grandchildren and four great grandchildren.