Born at Polwarth, Berwickshire, he was raised as a strict Presbyterian, and after a term of law study at Paris he became a member of the Scottish parliament in 1665 as shire commissioner for Berwickshire, where he at once took a foremost place as defender of the Covenanters. He went so far as to bring imprisonment upon himself, and on being freed was suspected of complication in the Rye House Plot, so that he was forced to remain in hiding until he could escape in disguise to the Netherlands. There he joined Archibald Campbell, 9th Earl of Argyll and embarked with him on the unsuccessful 1685 expedition to Scotland. Hume became a refugee with a price set upon his head; but he once more escaped abroad and lived at Utrecht under the name "Dr. Wallace," professing to be a Scottish surgeon. He returned with William of Orange at the Revolution of 1688, and once again joined the Scottish parliament as the commissioner for Berwickshire until becoming Lord Polwarth in 1690.