On 17 March 1761 Steuart was made a cornet in the 1st (Royal) Regiment of Dragoons (General Conway's regiment), through the influence of his father's friend Lord Barrington.
From 1764 to 1766 he travelled in France and Germany, studying the cavalry tactics and organisation there, and then was appointed to command a troop of the 5th (or Royal Irish) Regiment of Dragoons, joining the regiment in Ireland.