On 3 February 1789, Sanguszko began service in the Crown Army as a captain in the . On 5 October 1789, he was promoted to major, and on 28 April 1792, to vice-brigadier. He was a representative of the Lublin Voivodeship in the Great Sejm of 1788–1792. Sanguszko partook in the War of the Second Partition where he fought at the Battle of Zieleńce as a cavalry brigadier, receiving the Virtuti Militari in its wake on June 23. On 29 July 1792, the 24-year-old Sanguszko was promoted to brigadier, having taken over the command of the whole brigade on June 25. As the Targowica Confederation had won, he joined the Imperial Russian Army to save his goods and possessions from confiscation.