Originally Posted by
catocony
If she's inadmissible to Canada due to the conviction, then she won't be able to make the connection. It's just like doing an international-international transfer in the US. You need to be admitted just like everyone else, even though you'll only be here for a couple of hours.
If you're transiting through Terminal 1, that's simply not true.
If you're flying International to US, as you come off the plane, you'll follow the signs for US connections. You won't even see CBSA. You'll head directly to a desk where US CBP officers admit you to the United States, as YYZ has US preclearance.
Terminal 3 doesn't have such channels. I believe that all travelers arriving internationally at T3, whether intending on staying in Canada, or connecting to the US or internationally, must be landed by CBSA. (Note - this information about T3 might be out of date. It's been years since I flew through T3.)
Now, if I were inadmissible to Canada, I still might not choose this itinerary, even if connecting through T1. What if the YYZ-USA flight gets cancelled? I'd rather not spend the night in a holding cell at Pearson. But if everything goes smoothly, you'll probably be ok. Even if you were inadmissible to Canada, what would they do? You would be deported to the US (assuming that's your home country), probably on your already scheduled flight. There just might be a few uncomfortable hours with CBSA in between.