Borthwick was the eldest son of Sir William Borthwick "the younger", 2nd feudal baron of Borthwick, castellan of Edinburgh (Sir William Borthwick of that Ilk), and his wife Bethoc Sinclair of Orkney, daughter of Henry Sinclair, 1st Earl of Orkney.
His father Sir William de Borthwick junior, knight, had accompanied Henry, Bishop of Aberdeen, William, Bishop of Dunblane, John, Abbot of Melrose, James, Prior of Saint Andrews, John, Abbot of "Balmurynach", Sir William de Hay, knight, Master Thomas de Myrton, Master Edward de Lawedre, and Master John Stenes, all as Scots" ambassadors, with 50 attendants, setting out for the Court of Rome. The Safe-conduct is dated June 9, 1425 and is given until the following Easter. lieutenant is mentioned that the young William was sent in 1424 as hostage among those who replaced king James I in England.
In either 1424, 1438 or June 12, 1452 he was raised to the Peerage of Scotland as a Lord of Parliament with the title Lord Borthwick.
He sat in parliament between 1455 and 1457.
