He was the son of Adrianus de la Rey en his wife Adriana Wilhelmina, born Van Rooyen. In 1848 they moved to the farm Welverdiend in the present district of Wolmaransstad. As a child De la Rey received very little formal education but he never saw this as a hindrance as his parents' teachings and his natural intelligence gave him a sound foundation. Soon after the discovery of diamonds the family moved to Kimberley where de la Rey became a successful transport-rider before settling on a farm in the district of Lichtenburg. In 1876 he married Jacoba Elizabeth Greeff. Ten children were born from this marriage. The newly-weds moved to the farm Manana where De la Rey became so successful that he was able to buy the farm Elandsfontein near Lichtenburg were he lived to his death.
He soon showed signs of military leadership. As a nineteen-year old he took part in the Basuto War of 1865 and also as a field-cornet in the Sekhukune War of 1876. He was used by the government as a surveyor of farms and as the Native Commissioner in the Western Transvaal. As transport rider, surveyor and Native Commissioner he got to know the Western Transvaal intimately which would greatly contribute to his military leadership during the Anglo-Boer war. From 1893 he was a member of the Volksraad were his calmness and sense of fairness had a great influence on his fellow members.
During the First War of Independence (1880-1881) De la Rey took part in the siege of the British Fort in Potchefstroom. In 1885 he was elected as Lichtenburg's commandant.
On the eve of the Anglo-Boer War Cmdt.-Gen. Piet Joubert appointed De la Rey as Gen. P.A. Cronj's advisor on the Western Front. It was extremely difficult for De la Rey who was against the war in the first place to be responsible for the first skirmish of the war at Kraaipan where he had to derail an armoured train. On 12 October 1899 he captured twenty six British soldiers as well as three guns, a number of rifles and ammunition.
Cronj and De la Rey then had a difference of opinion regarding the siege of Mafeking as De la Rey did not agree with the idea of besieging Mafeking. On 19 October 1899 De la Rey was appointed as general with the order to besiege Kimberley. De la Rey first clashed with Lord Methuen at the battle of Graspan (Rooilaagte) op 25 November l899. During the battle of Modder River, that took place three days later, De la Rey was wounded in his shoulder while his eldest son Adriaan was killed in action.
To change Cronj's mind regarding the tactics to be followed at Magersfontein, De la Rey invited President M.T. Steyn to visit the western front. The result was that Cronj's approved the plan. The burghers were entrenched between 150 and 300 metres before the Magersfontein hills. De la Rey however was not present at the battle of Magersfontein (10 - 11 December 1899) as he had left a few days before the battle for Riverton north of Kimberley, to recover form his shoulder wound.
After the surrender of Cronj at Paardeberg on 27 February 1900, De la Rey desperately tried to stem the British tide at on 10 March 1900 at by Abrahamskraal (Driefontein) but without any success. From 28 May 1900 until 29 May 1900 De la Rey and Roberts once more clashed at the battle of Doornkop, near Johannesburg.
During a council of war at Balmoral during June 1900, De la Rey was instructed to reclaim Western Transvaal from the British. On 11 July 1900, he defeated Col. H. Roberts at Silkaatsnek. A number of burghers who had already laid down their weapons joined De la Rey's forces again. De la Rey now concentrated on isolated British units. It was important to de la Rey to once more establish the Boers' authority in the area and he therefore launched an efficient military re-organization of the area.
On 3 December 1900 he captured 126 wagons loaded with clothing, boots and Christmas delicacies from the British at Buffelspoort. He and Gen Beyers followed-up this success with a victory over the British forces under Gen Clements ten days later at Nooitgedacht. The official British losses were 638 while seventeen burghers were killed and sixty one were wounded.
De la Rey divided the commandos on the western front in smaller units and placed the Rustenburg and Krugersdorp commando's under his personal control. In 1901 several British commanders i.e. Methuen, Dixon, Cunningham and Kekewich were sent to capture de la Rey, however without any success. It was during this period that De la Rey developed his famous charging tactic which resulted in many losses on British side. In stead of dismounting the burghers developed shooting from the saddle during a charge into a fine art. The victory at Ysterspruit (25 February 1902) is proof of the success of this tactic. This resulted in his nickname the "Lion of Western Transvaal". Lord Methuen was wounded and captured during the battle of Tweebosch on 7 March 1902. De la Rey released Methuen so that he could receive medical attention.
De la Rey found the initial peace conditions unacceptable. On 29 May 1902, however he, with a short speech managed to persuade the burghers and General C R de Wet to lay down their arms. His reasoned that his commando's could carry on with the war but that the conditions in the rest of the country were rather bad. He was one of the co-signatories of the peace agreement on 31 May 1902
During the war De la Rey's wife assisted him in a very special way. During the first phase of the war she regularly visited him on the front and from 1 December 1900 she kept in close contact with him when she and the children wandered about in the veldt for nineteen months.
After the war he accompanied Generals Botha and De wet to Europe where they collected funds for the reconstruction of the country. He also travelled to India to persuade the diehards amongst the prisoners-of-war to sign the oath of allegiance. He became more and more involved in politics. He became a member of the Transvaal Parliament, a delegate at the National Convention, a member of the Senate of the Union etc. In 1914 De la Rey was in charge of the Government forces during the strike in Johannesburg. When the First World War broke out during the same year he did not agree with Gen. Botha's idea of attacking German South West Africa on behalf of Britain. He decided to attend a meeting of rebels in Potchefstroom. On the same day that Beyers resigned as Commandant-General of the Active Citizen Force he and De la Rey traveled by car form Pretoria to Potchefstroom. The driver did not stop at a roadblock set up for the Foster gang and he was killed by a ricochet on 15 September 1914.