McDonald and other prisoners were then confined for some time in the jail in Concord. While there, another British soldier was brought in, Robert Gaul, who had deserted from the 43rd Regiment of Foot in July 1774; Gaul had refused to serve in the Massachusetts militia and attempted to return to Boston but was caught as a deserter and was taken to the jail with hands tied. Gaul stayed only one night before being taken to Cambridge for trial. Some days later, Gaul showed up again, having escaped. A British serjeant, also captive in Concord, gave Gaul a pass with an American general's signature and sent him on his way to try to return to British service.
He had to wait for a ship to take him to home to Great Britain, and before he did he was able to come to the aid of a fellow soldier. Robert Gaul, the man he'd met in Concord jail who had escaped from Ameircan militia service, had been captured in May 1777 with a party of rebels in a house in New Jersey. He was brought to New York and put on trial for deserting and bearing arms in the rebel service. Gaul pleaded that he had been seduced to desert by inhabitants of Boston in 1774, but had refused to serve in their army and spent much time in prison because of it. Only recently had he agreed to enlist, to escape the deprivation of captivity, but when his corps was engaged by the British, he hid in a swamp until he could surrender.
Gaul called upon two British soldiers he'd met while in captivity to testify that he'd repeatedly expressed a desire to return to service. McDonald was one of the witnesses, who related the information about meeting Gaul in Concord jail and concluded his testimony by telling the court that Gaul "always shewed great contrition for having deserted His Majesty’s Service, and seemed very desirous of returning to the Regiment." Gaul was found guilty of desertion, but was spared capital punishment on the basis of McDonald's and another soldier's testimonies.
William McDonald returned to Great Britain, appeared before the pension board in Chelsea, and received his reward in October 1777. At only 32 years of age, however, he still had some fight left in him. In the 1790s, a rapid expansion of the British military in response to conflicts in Europe led to the raising of many new corps for local defense. McDonald enlisted in the Strathspey Fencibles, a regiment raised in his native region of Scotland for service only within the confines of that country. He served until the corps was disbanded in 1799, when he returned once again to the pension rolls.