This is another installment in my ongoing series on the American invasion of Canada in 1775.
Governor Guy Carleton made Montréal his headquarters after the Americans began to invade Canada. The colony’s chief defense was Fort Saint-Jean on the Richelieu, where the bulk of the 7th and 26th regiments was garrisoned. Once that fort was besieged, Carleton placed his hopes in raising a large body of Canadian militia to relieve the fort, or to at least prevent the Americans from making any further inroads into Canada. However, the rebellion of Canadian forces in the Richelieu valley and forays made by Colonel Ethan Allen and Major John Brown dashed Carleton’s hopes. On September 21, 1775, he bemoaned  that “A few days ago I had hopes of assembling a corps on the Sorell [i.e., the Richelieu river] and another at La Prairie, either of which might have saved the province for this year, but the friends of rebellion dissipated both by their intrigues and lies” (Allen helped capture Carleton’s agents on the Richelieu, and Brown captured La Prairie; cf. here). He determined, however, to “spin out matters as long as I can in hopes that a good wind may bring us relief” (i.e., until reinforcements should arrive from Boston or Britain).
In practice, this meant that Carleton remained holed up in Montréal while awaiting events. Carleton had with him a detachment of the 26th Regiment, some Indian Department officers and [Guy] “Johnson’s rangers,” and a small number of Indians. Montréal was a walled city, but Carleton took little comfort in this as the walls were “extensive and defenceless.” The town’s primary defense was its Canadian and English militia. The upper class of Canadians seemed dependable; already a number of these men were serving with the garrison at Fort Saint-Jean. The lower class, however, were at best ambivalent in their sentiments, and quite a few of the English citizens of Montréal had strong ties to the Thirteen Colonies and preferred the American cause.
Meanwhile, Colonel Ethan Allen was heading his way.
Allen distinguished him by co-leading the successful attack on Fort Ticonderoga. However, he afterwards led a meaningless expedition into Canada that almost got him and his command killed or captured [see events of May 17 and May 18, 1775]. (This misadventure in Canada was a primary reason why Allen was bypassed for the leadership of the Green Mountain Boys, when they organized as a regiment in the Continental Army).
A correspondent for the New York Gazette observed that “Allen is a high flying genius, pursues every scheme on its first impression, without consideration, and much less judgment. It was with the utmost difficulty, and through the greatest entreaty, that [Major] General [Philip] Schuyler permitted him to go with the army, knowing his natural disposition…” 
On September 18, Allen was at Saint-Denis, and contemplated capturing some British vessels anchored at Sorel. On the 20th he was at Saint-Ours, with, he claimed, 250 newly-raised volunteers. He boasted in a letter to Brigadier-General Richard Montgomery that he would raise hundreds, if not thousands, of men, and join Montgomery's army then besieging Fort Saint-Jean.  Soon thereafter, Allen marched to Sorel, but made no attempt to capture the British vessels. Instead, his force turned towards La Prairie. Allen could not pay, or feed, or arm his men, and only about 80 left the Richelieu valley with him. Those that did allegedly “plundered the Houses and Farms of the Gentlemen and Habitants, that had joined the King's Forces” “in every Parish on their Road.” 
On September 24, Allen’s party left Longueuil for La Prairie, marching along the stretch of St. Lawrence opposite Montréal. En route, he encountered Major Brown. According to Allen, “Col. Brown proposed that, "provided I would return to Longueuil, and procure some canoes, so as to cross the river St. Lawrence a little north of Montreal, he would cross it a little to the south of the town, with near two hundred men, as he had boats sufficient; and that we could make ourselves masters of Montreal."”  Allen quickly agreed, picked up 30 Americans that had been with Brown, and returned to Longueuil. His party crossed the river that night and landed in an area known generally as Longue-Pointe. 
1. Letter to William Legge, Secretary of State for the Colonies, dated September 21, 1775.
2. In Frank Moore (1859). Diary of the American Revolution: From newspapers and original documents.
3. The full contents of Allen's letter can be found here.
4. Here I am quoting a letter by Hector de Cramahé, lieutenant governor of the province of Quebec.
5. From Allen's narrative.
6. As shown in the map, Allen landed in a rural area considerably above the "village" of Longue-Pointe. The landing site was almost certainly within the modern-day Hochelaga-Maisonneuve district of Montréal.