Borthwick served as ambassador to England in 1459, his name was included in a safe-conduct or passport of 13 July as "William lord Borthwik" to travel to Newcastle with numerous other nobles, clerics, and a retinue of 200 attendants.
[2] On 11 April 1464 he was, with other senior peers, commissioned by James III of Scotland to negotiate with the ambassadors of Edward IV of England to conclude the treaty of York.
[5] In August 1471 he received a safe-conduct with several clerics and other nobles, and a retinue of 400 persons, allowing travel to England as "ambassadors and commissioners of the King of Scots coming to treat with the English commissioners".
His son and heir William Borthwick, appears as defender in a legal case, an action of debt on 4 July 1476.
[7] Alexander Nisbet, a writer on heraldic matters, published a 1484 charter referring to this Lord Borthwick and his spouse Margaret Hoppringle.