Born at Uppland, he was the son of Major Clas Anckarström and Brigita Charlotta Realize Stjerna.
He first joined the army aged 15, as a hantlangare in the artillery, and in 1745 travelled to France with lieutenant-colonel Leslie.
He was immediately employed in the Swedish Royal Suédois regiment, rising to lieutenant aged 18 (in 1747) and wounded at the 1746 Siege of Namur.
He returned to Sweden in 1748 and in 1750 (aged only 21) he was promoted to be a captain with the Nylands regemente.
In 1759 his Prussian captors allowed him to leave captivity to go to Sweden to take up a legacy from his father, who had died that year.