International flight to Toronto, connecting to US. When do I go through customs?

Toronto, same as if you were in Toronto and got a flight to Cincinnati.

And it is immigration or passport control. Custom is simply a checking of imported goods.

Here's the Toronto Pearson Airport connections page website, where you can learn all the details of your particular connection: https://www.torontopearson.com/en/connections

To clarify what Sam's post is referring to, major Canadian airports have "US preclearance." This means that when flying from Canada to the US, you go through US immigration and customs in the Canadian airport before departure; your arrival in the US is like a domestic arrival. This allows flights from Canada to arrive in airports like NY Laguardia and DC National, which do not have customs or immigration facilities.

So, you need to allow time between the arrival in Toronto and the departure for Cincinnati, but you won't go through anything upon landing in Cincinnati.

I would think you would go though immigration in Toronto. Not certain but when I fly from Pearson I go through US immigration in Toronto. It’s nice to land in the States and not have to line up at your destination.
Not sure but my best guess

Yes, you would have to preclear both US Immigration and Customs before boarding your flight to the US.

Yes, Toronto has had "US pre-clearance" since at least the early 2000s. You have to go through it before you can go to the boarding area of your US-bound flight. It can take quite a while (an hour or more) if there are many travelers ahead of you. The actual "drill" that you have to do involves showing your passport, verifying a photo of your checked luggage (if any), possibly a couple of other steps (I can't remember exactly) and takes about 15 minutes.

Thank you everyone so very much, this helps a great deal in planning my trip! Has anyone gone though the customs/immigrations in Toronto and how was it? I had read the Toronto airport explanation on this, but not having traveled very much, I really didn't understand it. So you all clarified very well and I thank you!

Did that several years ago as flying through Toronto was the cheapest ticket to Europe. It was not very trying, except that you had to also go through official TSA security at Toronto. A nice little cork screw we bought in Europe was confiscated because it had that little 3/4" blade for cutting off the seal on a wine bottle. Security in Duesseldorf had no problem with it, but TSA was adamant.

Sam, Thank you for mentioning TSA.
Elaine