He served with the army in Scotland, France and Flanders, and held a seat in the House of Commons of Great Britain from 1708 to 1715.
The first confirmed fact of his military career is that in 1707 he became a Lieutenant-Colonel in the regiment newly raised by Alexander Grant.
In a contest assisted by his opponent being jailed as a suspected Jacobite, Stewart was returned as the MP for Kirkcudbright Stewartry.
Released on parole, he was re-elected in the Stewartry, but negotiations with the French proved slow and he was not fully cleared until late 1711.
One of those present was the by-election winner Sir Gilbert Eliott, 3rd Baronet, of Stobs, who complained to Stewart for not voting for him.