Public resentment at a possible reprieve was such that a plot to murder Captain Porteous was hatched, and when the authorities heard of this it was decided to increase the guard at the Tolbooth prison. However, on the evening before this was due to happen, a large crowd of over four thousand gathered at Portsburgh, west of the city.

Making their way across the Grassmarket to the Cowgate and up to the High Street, the mob converged on the Tolbooth, where they were eventually able to overpower the guards. Porteous was dragged from his cell up the Lawnmarket to the West Bow and down to the Grassmarket, where he was lynched from a dyer's pole, using a rope taken from a local draper's shop.