Reading up a little on Mississauga, and when they amalgamated in the mid '70s, they chose a building at Hurontario and Dundas for city hall. And as this was when Square One was just a mall, and Hurontario and Dundas had a bunch of stores on all corners, I guess we could say that was Downtown.
Later on, they moved city hall to SQ1 area and of course built a new one that actually looks like a city hall in 90s. And SQ1 area is surrounded by condos and small office buildings so that's Downtown now as the city itself has stated. Basically shifted up one major road north to Burnhanthorpe. However, no one in Sauga actually says "I'm going to Downtown Mississauga". They might say "I'm going to Square One" even if they don't intend to go to the mall.
It's like that in Vaughan, Caledon, Halton Hills, Pickering and Ajax. There's no downtown. Though in these places the city hasn't specifically said there actually is a downtown like they have in Sauga.

insauga.com

adiseshan shankar / Alamy Stock Photo
http://www.squareonecondos.ca/
This is what Hurontario and Dundas looks like now. Sketchy is a good word to sum it up. Used to play pool around here a lot and you will still see plenty of sketchy people hanging around. And up until a few years ago, there was also a trailer park just to the west of there, hidden behind some businesses. Lease was up and they had to get the hell out of town. Thankfully plans are in the works for re-development of the neighbourhood. Condos will slowly be going up. It makes so much sense with Cooksville GO station being right there.

Rob Beintema/Metroland

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This so called downtown may have the city hall and tall buildings, but lacks the charming main street lined with quaint stores on a water setting like neighbouring Oakville and Burlington, also on Lake Ontario.
Mississauga does have that neighbourhood in Port Credit at Hurontario and Lakeshore, where the Credit River meets Lake O. And Streetsville has the same thing but upstream on the Credit River.
But, with a population nearing 800k, downtown might as well be where the tall buildings are. It's just unconventional for a big city's downtown to not be on the water.

James Hackland / Alamy Stock Photo
https://urbantoronto.ca/forum/threads/st...-st-mississauga-dezen-6x3-7x7-3x8.13599/