The outbreak of war in America brought them a second opportunity to volunteer. Besides the regiments of the regular army sent to America, a composite corps was formed of volunteer soldiers from the three regiments of Foot Guards that traditionally provided security for the monarchy and government in London. 15 volunteers were drawn from the private soldiers in each of the 64 companies that comprised the Guards regiments, in addition to officers, non-commissioned officers, drummers and fifers. This detachment operated in America as the Brigade of Guards; it included a grenadier company composed of soldiers from all three of the Foot Guards regiments. McChesnie and Mouncey were both corporals in this company. On the night of 1 August 1779 they were making their way through the dark streets of old New York after a walk on the Battery at the south end of Manhattan island.
Also out for a walk that night were two young officers, Lieutenants John Wallace of the 22nd Regiment of Foot and James Drury of the 57th Regiment of Foot. They were walking towards the Battery between 9 and 10 o'clock when they parted company with each other. Moments later, Lt. Wallace heard a commotion. He drew his sword and ran to the noise where he found Lt. Drury struggling with two soldiers of the 17th Light Dragoons. The dragoons had hold of Drury, but released him after receiving several strokes from the flat of Wallace's sword; the two officers secured the dragoons and set off towards the provost with them.
This is when the two corporals met up with the two officers. Apparently there were other soldiers hovering in the darkness, perhaps drawn by the commotion, because the officer perceived that they had encountered a party of about 10 grenadiers. Lt. Drury directed the soldiers to take custody of the two dragoons and take them to the main guard. McChesnie, a with nearly 20 years in the army, responded that he could not take them without first getting their names and the charges against them; without this information the guard would not receive them. The officers, already agitated from their scuffle with the dragoons, viewed this as a flippant response. Wallace (who, it bears noting, was 19 years old; Drury was probably only a few years older) told McChesnie that he and the Brigade of Guards were rebellious rascals and, gesturing with his sword, threatened to imprison McChesnie if he did not comply with orders. McChesnie snatched the sword from Wallace's hand and responded that he would not be taken prisoner. Drury seized McChesnie by his bayonet belt and demanded his name, which McChensie readily gave. Wallace, in the mean time, moved behind McChesnie, grabbed the sword again and wrested it free. More threatening words were exchanged. Wallace asked for help from another soldier who not only refused but drew his bayonet. McChesnie's bayonet belt gave way. A scuffle ensued in which the officers received blows, but within moments all of the soldiers cleared off into the night leaving the two officers with only McChesnie's bayonet belt, an unidentified hat, and extremely wounded pride.
The young lieutenants took the belt to the commander of the Brigade of Guards who were able to trace it to McChesnie. The long-serving corporal was brought before a general court martial the following week on charges of abusing the officers and threatening Lt. Drury's life. The officers gave their version of the story, characterizing McChesnie as drunk and insolent, while Mouncey corroborated McChensie's testimony that they were sober and compliant, but that the officers became belligerent when asked for necessary information. An officer gave a favorable character witness. In a "his word against mine" situation like this a court composed of army officers had little choice but to find in favor of one of their own. Corporal McChensie was sentence to be reduced to the ranks and receive 1000 lashes.
Thomas McChesney continued his career as a soldier. After being discharged from the Foot Guards he served in several garrison corps in Great Britain, taking his final discharge in 1796 at the age of 58. Joseph Mauncey also stayed in the army, remaining the 1st Regiment of Foot Guards until 1787. Both received pensions.