Rehnskiöld grew up in Swedish Pomerania and studied at Lund University under philosopher Samuel von Pufendorf. He entered Swedish war service in 1673 and participated with distinction in the battles of Halmstad, Lund, and Landskrona during the Scanian War, where he was appointed Lieutenant-Colonel and Adjutant-General. After the war, he was commander of several regiments, observer and tutor to Duke Frederick IV during the Nine Years' War, and Governor-General of Scania.