He was knighted before 18 September 1439 and had a Safe-conduct to pass through England dated 23 April 1448, when he accompanied Lord Chancellor Crichton's Embassy to Flanders, France, and Burgundy.
He was a Privy Councillor by 11 July 1458 and made a Lord Auditor in 1469/70.
He again had a Safe-conduct to travel to England as Ambassador on 16 March 1472 (1471/2), and once more on 21 April 1473.
He married twice: (1), before 8 January 1436/7, Margaret, only child of John Stewart, 2nd Earl of Buchan and Constable of France, by his spouse Elizabeth, only daughter of Archibald Douglas, 4th Earl of Douglas; (2), before 8 January 1460/1.
The 3rd Lord Seton died shortly after 15 July 1478 at the Black Friars, Edinburgh, and was buried there.