Allen and Brown soon linked up with a body of pro-American Canadians organized by James Livingston. These men kept watch on the British garrisons at Fort Saint-Jean and Fort Chambly, and awaited the advance of Schuyler’s army. On or about September 11, Allen, however, couldn’t resist the temptation to capture five royal artillerymen  travelling between the two forts.
At the time, Livingston was operating with a small party of men on Île Sainte-Thérèse, between Fort Chambly and Fort Saint-Jean. He lamented to Schuyler, “I have begun a war,” because of Allen’s action. Livingston could scarcely provide his volunteers with provisions, arms, or ammunition. No matter. The next day, the British sent out two bateaux from Fort Chambly, one of which allegedly contained 20 armed men, and the other stores for Fort Saint-Jean. As the British neared Île Sainte-Thérèse, Livingston’s men blasted them with musket fire, killing or wounding as many as a dozen men, and sending the dazed survivors fleeing to neighboring Île Sainte-Marie. Both boats were captured.
Not long after, Livingston retired to Pointe-Olivier, downriver from Fort Chambly, and Allen and Brown returned to Schuyler’s camp. Schuyler sent out Allen and Brown a second time in preparation for the Americans final push against Fort Saint-Jean. On September 17, Brown, acting with some Canadian volunteers, intercepted supplies heading for Fort Saint-Jean. The next day, Allen and Livingston captured two British agents at Saint-Denis. Soon after, Allen and Brown occupied key towns on the Saint-Lawrence. Brown took up post at La Prairie, and Allen advanced to Sorel.
Meanwhile, the American army, now led by Brigadier-General Richard Montgomery, lay siege to Fort Saint-Jean. While this occurred, Livingston, with a force hovering around 300 men, continued operations on the Richelieu. Livingston’s chief responsibilities were to protect Montgomery’s northern flank and to help stop any breakout attempt from Fort Saint-Jean. The poorly-armed Canadians demanded, and eventually received, two 4-pounder field pieces.
Livingston’s War (click to enlarge). The part of the Richelieu in which Livingston’s Canadians operated during September and October, 1775, is indicated by the blue line (i.e., from Fort Saint-Jean to the parish of Saint-Denis). In mid-September, at least, Livingston’s headquarters was at Pointe-Olivier, near Fort Chambly. The British garrisons at Fort Saint-Jean, Fort Chambly, and Montreal are indicated (cf. Carleton Defends Canada), as is Montgomery’s American army besieging Fort Saint-Jean and Brown’s and Allen’s late September advances to the Saint-Lawrence.
Mostly the Canadians were allowed to act with impunity along the Richelieu. One known exception appears in a letter by Lieutenant-Colonel Samuel Mott of Connecticut (Montgomery’s chief engineer). He wrote that:
“On the 3d instant there was a severe engagement between the French Whigs on one side, and the French Tories and Regulars on the other side, at Chambly, about thirteen miles from this place. The Tory party had the advantage, as they fired about fifty cannon-shot on our Whigs, when they had only small arms to defend with. They lost several men on each side. The Whigs maintained their ground.”
In the absence of significant threats, Livingston’s men were able to contribute to the siege of Fort Saint-Jean, and attack Fort Chambly.
First, Livingston’s men erected a breastwork northeast of Fort Saint-Jean on the estate of Moses Hazen. The British attacked this force on October 4 with a heavily armed row galley . The Canadians responded with musket fire and their little 4-pounders. The British eventually withdrew after failing to make any impression on Livingston’s men (according to Mott, they lost “only one man, slightly wounded with a grape-shot”).
Soon thereafter, the Canadians moved their cannon closer to Fort Saint-Jean, and on the 13th they were joined by an American gun section consisting of two 12 pounders. The combined battery sank the British schooner the Royal Savage and inflicted a number of casualties among the garrison.
In mid-October, Livingston was granted permission to attack Fort Chambly. The attack was begun by his men, Major John Brown and around 50 Provincials, a 9-pounder cannon, and Lieutenant Johnston and two privates from Lamb’s Artillery company. The bombardment began on October 17. The next morning, a second 9-pounder joined in the firing. After a day and a half of cannonading (i.e., by midday on the 18th), a small breach was made in the fort’s outer wall. At that point, the 88-man British garrison, consisting chiefly of men from the 7th Foot, agreed to surrender. 
1. They were Thomas Goone, gunner, and Matthew Bell, John Boetle, Osburn Frederick, and Robert Knox, matrosses, of Captain Jones' s Company, 4th battalion, Royal Artillery
2. According to Major Henry Livingston of the 3rd New York, the galley had one 24-pounder in the bow, and 1 four pounder and some swivel guns on each side.
3. The complete list of men can be found here.