21 May 1910
A former member of the Volksraad of the South African Republic (Transvaal), representing the Vryheid district in 1897 and also Boer general during Anglo-Boer War 2, Louis Botha, was instructed by Governor-General Herbert Gladstone to constitute a government in terms of the South Africa's Act. He then became the first Prime Minister of the Union of South Africa, which was to be inaugurated on 31 May.    
References

Wallis, F. (2000). Nuusdagboek: feite en fratse oor 1000 jaar, Kaapstad: Human & Rousseau |

South African History Online, 'Louis Botha', [online], available at www.sahistory.org.za (Accessed: 21 May 2013)