Scottish Presbyterian who was a leading anti-Royalist during the English Civil Wars between the Royalists and the Parliamentarians. Later he became an official in Oliver Cromwell's Commonwealth regime. He was known to his contemporaries as petulant and quarrelsome.

Trained in law, Johnston was a principal author of the Scottish National Covenant (1638), which denounced King Charles I's attempts to impose Anglican forms of worship on the Presbyterian Church of Scotland. In his judgment, episcopacy was “that great-grandmother of all our corruptions.” During Charles I's (Charles I) conciliatory visit to Scotland in 1641, Johnston was knighted and appointed a lord of session with the courtesy title of Lord Warriston. From 1644 to 1646 he assisted in the coordination of joint military operations of Covenanting and Parliamentary armies against the forces of Charles. He was made lord advocate in October 1646.

When the moderate Covenanters (Covenanter) joined Charles against the Parliamentarian Independents (radical Puritans) in 1647, Warriston led the extremist Covenanters who opposed the alliance. Nevertheless, after the Independents executed Charles and formed the Commonwealth (1649), Warriston reluctantly backed Charles I's son and successor, the exiled Charles II, who had agreed to honour the Covenant. Warriston was ousted from the government when Cromwell (Cromwell, Oliver) conquered Scotland in 1651, but he accepted from Cromwell in 1657 the offices of lord clerk register and commissioner for the administration of justice in Scotland. In 1658 he sat in Cromwell's Parliament, and in the following year he sat in Richard Cromwell's Parliament. In 1663, however, three years after the Restoration of King Charles II, Warriston was hanged for his previous anti-Royalist activities.