Van Deventer was born in Ficksburg in the Orange Free State. He began his military career in the Transvaal republican forces in Pretoria on 21 February 1896, and fought in the Anglo-Boer War (1899-1902). He was physically a very big man standing almost 2 m tall. A serious wound at the end of the Anglo Boer War affected his ability to speak. Van Deventer was skilled at guerrilla tactics.
He returned to military service in World War I, and commanded a Union Defence Forces formation in the South West Africa Campaign (1914-1915). In the German East Africa Campaign (1916-1918), he commanded a South African Overseas Expeditionary Force mounted brigade, then a division, and finally (1917-1918) all the British imperial forces in the region (though, allegedly, he could hardly speak English). He was knighted twice for his services.
After the war, he was a part-time inspecting officer. He was made a Grand Officer of the Portuguese Military Order of Aviz in 1921. In 1922, he commanded a mounted brigade in operations to crush the Rand Revolt on the Witwatersrand. He married Maria Cornelia Snyman, born 5 May 1890. He died later in Pretoria in 1922.
- Anderson, Ross (2003). "J C Smuts and J L van Deventer: South African Commanders-in-Chief of a British Expeditionary Force". South African Journal of Military Studies. 2. 31.
- "Futuristen - historier fra nettet". Archived from the original on 11 May 2008. Retrieved 27 November 2012.
- "No. 32428". The London Gazette (Supplement). 19 August 1921. p. 6570.
- Van Deventer W.D Dr (1993) Van Deventer stamboom en Agtergrond
- Nothling, C.J. (1994). Suid-Afrika in die Eerste Wereldoorlog.