We do not know when Derring joined the army, but he was a corporal in the light infantry company by the time the 47th regiment arrived in America in 1773. Early the following year he was reduced to a private soldier. A reduction in rank can be interpreted as evidence of bad discipline, but men were reduced for other reasons: illness that prevented them from performing their duties, promotion of an even more-qualified man, detachment from the regiment for any of number of reasons, just to name a few possibilities.
In 1776 Derring was transferred from the light infantry into one of the eight battalion companies, a sign that he lacked either the fitness or the discipline for the aggressive activities of the light infantry. A few months later he transferred to another battalion company, and in September 1776 was again appointed corporal. This improvement in his situation did not last long. The following August he was on Burgoyne's campaign to Albany when he most certainly ran into disciplinary troubles. He was reduced again to private, and along with another soldier of the 47th was tried by a general court martial for "robbing Mr. William Johnson at Fort Edward on 7 August." We do not have the date of his reduction, but it appears to have occurred before the crimes was committed; possibly he had been reduced earlier in the year for another infraction. The trial was held on 10 August, and a guilty verdict was announced in general orders on the 16th. Both Derring and the other man were sentenced to receive 1000 lashes each. We have no evidence of whether the punishment was administered in part or in full, or was pardoned altogether.
In October, John Derring became one of thousands of British prisoners of war incarcerated when Burgoyne's army surrendered at Saratoga. Although initially intended to be returned to Great Britain, the Convention Army (so named for the convention that describing the terms of capitulation) was instead held in barracks outside of Boston for over a year before being marched to the interior of Virginia and Pennsylvania. This long and frustrating captivity afforded ample opportunities to desert, particularly because soldiers were offered the opportunity to work in the country. Large numbers of British soldiers effected their escape, however, not to desert but to make their way to the British army in Rhode Island, New York or Canada.
Knowing John Derring's experience as a soldier, we'd expect him to be disaffected with the army and take the opportunity to desert. Instead, he managed to escape and eventually got in to New York where he joined the 57th Regiment of Foot. This could be viewed as simple opportunism, but Derring gave a deposition to a court of inquiry in which he briefly described his experiences. It is clear from his story that his escape was no simple task, and only through amazing tenacity did he effect it completely and make his way into New York. His deposition reads:
John Derring late of the 47th Regt. of Foot & now drafted into the 57th Regt. says that he made his escape from Prospect hill barracks on the 1st. of March 1778, but was apprehended & put into Goal in Easton, where he lay 9 Months, and was then moved to Philadelphia Goal, where he was confined two Years and four Months, and then with 18 other British Soldiers broke out of Goal, by digging with no other instrument than their knives, this being the seventh time he had made the attempt & after being 21 days in irons in a [illeg] for making these attempts: that on the 29th of May 1782 he arrived at New York; he therefore claims his pay (one Guinea excepted, which he received in Philadelphia Goal) and cloathing (except two [illeg] from the time he made his escape from the 47th Regt. at Prospect Hill to his being drafted into the 57th Regt. vizt from the 1st of March 1778, to the 29th of May 1782
This story sounds remarkable but it was one of dozens of depositions given by British soldiers who had gone to considerable lengths and endured immeasurable risks to get from captivity back to the British army. Many more did so but left no depositions or other details of their efforts. All this in a country where they could easily melt into local populations, find work, and settle. Clearly they were highly motivated to return to the army and eventually to Great Britain. While we do not know their actual motives, it is certain that life in the army was not unbearable - Derring and hundreds of other soldiers who had many other choices nonetheless returned to the service.
The muster rolls of the 57th Regiment would tell what became of John Derring, but we have not had the opportunity to examine them. When he joined the regiment, Derring could not have known that they would be sent to Canada at the end of the war in late 1783. Whether Derring was offered a discharge from the army, or went on to Canada with many of his fellow career soldiers, is left for future research to determine.