His father had been granted lands in Ireland by Charles II of England in lieu of arrears of pay due to him as a Cavalier officer during the English Civil War, and it was there that William Steuart grew up. He joined the English Army and became a Captain with the 1st Foot Guards before his promotion by William III of Orange to Lieutenant-Colonel of the 16th Foot. He was subsequently promoted to Colonel of the 9th Regiment following the Glorious Revolution. Before Stewart took command, the regiment had taken part in aborted attempt to relieve the besieged city of Derry under another Colonel John Cunningham who was dismissed by King William for his failure. Stewart's regiment was re-embarked as part of General Percy Kirke's second relief force for Derry.