Mason ran into some misfortune in America. In the closing stages of the battle of Princeton on 3 January 1777, Mason was among 77 men of the 40th taken prisoner. He may have been among those who surrendered at Nassau Hall, the central building of what would become Princeton University. Because he spent the next year in captivity he missed the October 1777 battle of Germantown where part of the 40th again found itself holed up in a building, this time with quite different results.
The Princeton prisoners were set to confinement in Connecticut. Typical of British prisoners of war throughout the American Revolution, they escaped in considerable numbers; several were advertised in newspapers throughout 1777. Stewart Mason did not make his break until April 1778, as evidenced by an ad placed by an American commissary of prisoners:
Made his escape from Windsor, one Stuart Mason a British prisoner, he is about 23 years of age small of Stature, fair complexion, a large scar under his chin. Whoever will bring said fellow to the goal in Hartford shall have a handsome reward and necessary charges paid by Ezek. Williams Com. Prisoners.
Mason made a clean getaway, probably into New York. On the 40th Regiment's muster rolls for the first half of 1778 he no longer appears as "prisoner with the rebels" and is once again a corporal.
The 40th Regiment was one of several sent to the West Indies at the end of 1778. The next rolls for the 40th were prepared on the island of Antigua in 1780. By this time Mason had become a serjeant, and he remained in this capacity when the regiment returned to New York for the final years of the war in 1781. He served in the regiment until 1795 when he was discharged and received a pension for 27 years of service. A career military man, however, he went back into the army on the staff of a barracks in Great Britain. He was finally discharged from that post and returned to the pension rolls in 1805.