I wrote this as a hypothetical "day in the life of..." kind of thing last year for the Museum of Ontario Archaeology. I wrote a whole series of them for each time period, but the 11,000 years ago one (Paleoindian Period) is the only one that's up so far. I don't want to send you the others since I wrote them when I was an employee of the museum, and I don't know how that works copyright wise and all that. I would like to volunteer some time for them to get the others in the series ready to post, but I have other things I need to do more urgently (and procrastinate like mad). We'll see though, maybe I can do something in the next couple months.

Explore the museum's website a bit too, especially under the "discover" tab. There are a few short videos by Chris Ellis, one of the foremost Ontario archaeologists, describing each of the three broad periods in precontact Ontario history. There are also some other links there that have an overwhelming amount of info, especially this site. It's poorly organized and outdated, but there's a lot there. And since you are asking specifically for southwestern Ontario I presume that you are in or near London. If you can, go to the museum. It is inexpensive to get in. To be honest, it's not a great museum and it has a lot of problems, but there are some good people there who are working on it, and they have some great artifacts. They also have a partially reconstructed village at the museum, which is the location of an actual 15th century Iroquoian village.

To answer your question a bit more specifically, the people who lived in this region shortly before direct European contact were an Iroquoian group we call the Neutral or Attawandaron. The French gave them that name since they were neutral in the wars between the five nations of the Iroquois (a confederacy of nations who lived mostly in upstate New York) and the Huron-Petun or Wendat, another Iroquoian group that lived in the Toronto and Lake Simcoe area. But not much is known about the Neutral. The French had first encountered them in the Niagara region, but the neutral had already abandoned southwestern Ontario at that time so this area appeared to be empty and "pristine" when the French first saw it for themselves (had the French come 100 years before they would have found a bustling and populous area with many villages around the Thames and its tributaries, and with fields as far as the eye could see).

So we don't know much about the Neutral, but we do know that they were Iroquoian and spoke an Iroquoian language, so we make assumptions that they lived in a similar way to the Huron, who we do know more about. What about before the Neutral? That is hard to say, and I'll admit here that Ontario is not my main focus and I'm not entirely up to speed on current ideas about populations movements. It is really hard to push ethnicity or group affiliation back very far, at least using archaeological data (oral histories are different and can help do that, but since the Neutral left this area it is hard to trace those stories). And although people in Ontario and neighbouring regions were largely sedentary after about A.D. 1000, there were still large migrations and movements and it is hard to know whether that happened in this region too, and there is some debate about that. We know that the Neutral were here after about A.D. 1400, but it's hard to know who was here before, and impossible to know how people saw their own ethnic identities.

My main source for all this is Before Ontario: The Archaeology of a Province, edited by Marit Munson and Susan Jamieson. It is a great source with chapters written by many of the biggest names in Ontario archaeology, and they are all very readable and informative for non-specialists. Neal Ferris and Gary Warrick have chapters dealing with southwestern Ontario in the more recent precontact period, and Chris Ellis has one dealing with the more ancient stuff (11,000 - 3000 years ago). The museum has a small bookstore and they stock this title as well as others that you may find interesting, if you make it there.

Edit: oh yeah, and you can ask this as well in /r/askanthropology. This question really falls as archaeology rather than history, and there is a greater concentration of archaeologists there. I'm not sure if anyone there is specifically an Ontario archaeologist, but there are some people who are familiar with the archaeology of eastern North America in general.

As /u/Pachacamac said, southwestern Ontario was home to the Neutral Confederacy at the time of initial European contact. “Attawandaron” is their Wendat (Huron) name and is a reference to the fact that speak a language similar to that of the Wendat themselves. “Chonnonton” may have been the name they had for themselves, or the Wendat name for one of the larger nations that made up the confederacy. This name means either “People of the Deer” or “Keepers of Deer.” I’ll be using this name throughout the post for the confederacy as a whole though.

There were between 5-10 nations that made up the confederacy, but sorting out which names mentioned in the historical record are nations, which are clans, which are villages, and which are synonyms is next to impossible. It’s been argued that the Chonnonton were really more of a “chiefdom” than a confederacy (with a singular leader at the top of the political pyramid) based on the description of Tsouharissen as the supreme leader of the nation during a visit by a French missionary in 1526. There was also the “Mother of the Nation” / “Fire Woman” who’s described as the female ruler of the confederacy. The modern Seneca historian Barbara Alice Mann gives her name as Kieuneka, but this might be the name of the town she lived in instead located on the Niagara River. I think it’s more likely that Tsouharissen and “Kieuneka” were the heads of the men’s and women’s councils for the confederacy, paralleling the roles of the Tadodaho / Adodarhoh and the Jigonsaseh of the Haudenosaunee (Iroquois), the more authoritarian roles that Europeans tended to ascribe to Native leaders.

Culturally, the Chonnonton described by the Wendat and the French who had limited direct contact with them as being similar to that of the Wendat themselves, with a few notable differences.

The Chonnonton were sedentary agriculturalists. They lived in 40 or so villages and towns consisting of longhouses surrounded by palisades. The Lawson Site in London, Ontario, is a famous example of a large pre-colonial Chonnonton town, with an estimated population of around 2,000 at its peak in the 16th Century (this is the one Pachacamac mentioned being partially reconstructed by the Museum of Ontario Archaeology). It would a bit like this, though here is the specific layout of the Lawson Site. An extended matrilineal family lives within each longhouse. The longhouse consists of a central corridor running its entire length. Every ten feet or so is a partition that separates the longhouse into compartments, each one housing two families.The ends of the longhouse were a bit flimsy than the rest of the structure because they’d be disassembled regularly in order to extend the longhouse further as the family grew and were used for storage in the meantime. This cutaway shows you the entrance, the storage room, and the first comparment of a longhouse. Also, in a Chonnonton longhouse, unlike a Wendat one, you might find the bones of the recently deceased being stored inside awaiting their final burial. Iroquoian towns moved periodically and as part of the ceremonies accompanying the move, the town’s dead are all placed in a single large ossuary.

Outside the village, there would be many acres of farmland. The Chonnonton grew the Three Sisters - maize, beans, and squash - like almost everyone else in the Eastern Woodlands at the time. There would be several varieties of each, for example, corn used to make flour was a different variety than that used for other purposes. They also grew sunflowers and tobacco. Women did the bulk of the farming, though men would help with clearing the fields initially and with the harvest. The fields would be farmed for 4-8 years, then allowed to fallow. During the fallow period, they still remained productive however, as they became home to various berry-producing undergrowth (strawberries, blackberries, etc.) and later by fruit and nut-bearing trees (plums, mulberries, cherries, walnuts, etc). While Native American farmers generally didn’t practice crop rotations, this manipulation and utilization of forest succession filled the same niche.

While the women tended the fields and gathered wild plants, the men focused on hunting. Deer would be the main source of animal protein, and were abundant in the area. One of the noted differences between the Wendat and the Chonnonton was that the Chonnonton kept deer in pens (hence the name “Keepers of Deer”). When Daillon visited Ounontisaston (Tsouharissen’s town) in 1626, he observed three large deer pens outside the town. There’s some debate over whether these deer were penned from a recent game drive or whether the Chonnonton were actually engaged in some nascent domestication of deer. Black bears were also an important food source; most of the other animals hunted are very lean, so black bears provided much needed fat in the diet. Hunting was seasonal, however, with most of it being done in autumn and early winter. Of course, if the Chonnonton were keeping deers penned year-round, they might have relied on them more regularly. Ultimately though, farming provided most of a person’s calories over the year, and deerskin for making clothing was perhaps just as - if not more - important than hunting them for meat.

Summer is the season for travel. Assuming there is a council and Tsouharissen isn’t some sort of despot, this is when it would take place. This will be a large public event. Even people who aren’t members of the council will attend to observe the proceedings. This will include women, who will be observing to see if the representative for her matrilineal clan is living up to his expectations. If “Kieuneka” is the head of a women’s council, at least some of the women council members will be in attendance to keep well informed about what the men’s council is up to and what it’s opinions on various topics are. If the Chonnonton are like the Haudenosaunee in this regard, they will also have specific delegates whose job it is the liaison between the two councils (a man chosen by the women’s council to speak for them at the men’s council and a women chosen by the men’s council to speak for them at the women’s council).

During the summer traders would also be out and about. The Wenro, one of the few nations of the confederacy that we know with certainty, were particularly famous traders and actively pursued trade relations with the French. They seem to have been relatively successful at this, especially considering how little historical interaction there was between the Chonnonton and Europeans, since there’s quite a lot of European trade items (metal pots being a particular favorite) in late Chonnonton grave goods. The Chonnonton also had rather strong trade relations with the people of Chesapeake Bay, either directly or through an intermediary like the Massawomeck, another large Iroquoian confederacy (who are probably the people known archaeologically as the Monongahela, located roughly between what’s now Pittsburgh, Pennsylvania, and Charleston, West Virginia).

Finally, summer was the typical time for war. While the Chonnonton stayed out of the Wendat-Haudenosaunee conflict as long as possible (to the point of throwing the Wenro under the bus to avoid pissing off the Haudenosaunee), they weren’t averse to warfare. They formed an alliance with the Odawa (located along the northeast coast of Lake Huron) to fight the “Fire Nation” (located in the vicinity of Toledo, Ohio / Detroit, Michigan), which is either synonymous with the Mascouten or a confederacy which included the Mascouten. That war ended in 1646, when the Chonnonton brought home some 800 captives.

In 1650-1, after the Haudenosaunee defeated the Wendat, they turned their attention to the Chonnonton. The Chonnonton allied with the Susquehannock Confederacy in central Pennsylvania. While they had some success repelling Haudenosaunee attacks thanks to the alliance, by 1653, the Chonnonton had been defeated. The larger towns were dispersed and their people assimilated into Haudenosaunee towns. Smaller towns and villages were overseen by a Haudenosaunee official (probably the same position held by the “half-kings” that oversaw Shawnee and Lenape communities in the Ohio Valley leading up the French-and-Indian War). French records stop mentioning the Chonnonton by 1671, though the biography of Ely Parker (a Seneca brigadier general for the Union during the American Civil War) briefly talks about John Kenjockety (aka Skendyuhgwadih, Beyond-the-Multitude) a prominent Seneca man who died in 1808 and still identified with his Chonnonton ancestors.

The Haudenosaunee held the region for about 50 years, before they were defeated in their war against the French-allied Anishinaabe and forced to accept the Great Peace of 1701, in which the Haudenosaunee gave up their lands north of the Great Lakes and west of the Scioto River. After this southwestern Ontario was settled by the Odawa, Potawatomi, Ojibwe, and Missisauga. The Haudenosaunee would return to the area after the American Revolution. The war had broken their confederacy and there were two attempts to reform it - one of which became the Six Nations of Grand River near Brantford. The other remained in New York, but moved the grand council from Onondaga to Buffalo.