He followed the same occupation. Being required in 1678 to take arms on behalf of the government, he declined, and was obliged to leave Stirling to avoid imprisonment. He went to Glasgow, Falkirk, Bo"ness and other towns, pursuing his calling as he could find opportunity.
But, returning to Stirling, took part in a skirmish with dragoons at Ballyglass, near Fintry, on 8 May 1679.
On attempting to re-enter Stirling after the fight he was apprehended and thrown into prison, where he lay until in July following he was removed with a number of other prisoners to Edinburgh, and confined in the Greyfriars churchyard. Here in a small walled-in piece of ground nearly fifteen hundred prisoners were strictly warded, most of whom had been taken after the battle of Bothwell, and among these Garnock exerted himself to prevent them taking the "test." He was removed on 25 October for judicial examination, and, on declining to answer certain incriminatory questions, was incarcerated in the Tolbooth of Edinburgh.
Here he remained, refusing all overtures for compliance, until 7 October 1681, when he was tried before the privy council, and for declining the king"s authority was found guilty of treason, and condemned to be executed along with some of his fellows on the 10th of the same month. The sentence was carried out at the Gallowlee, between Edinburgh and Leith, his head and hands being cut off and placed on spikes at the Pleasance port of the town.
They also took down the heads of Garnock and the others, in order to place them beside their bodies.
When in prison Garnock wrote an account of his life, from the manuscript of which John Howie, in his ‘Biographia Scoticana, or Scots Worthies,’ gives several extracts. His dying testimony is printed at length in the ‘Cloud of Witnesses’.