Americans originating in Montreal to fly to Europe -- Any hurdles?

You'll be fine. Lots of other posters here have done similar things, and you will only need your passport. Of course, you need to allow time for crossing the border by car, so you don't miss your plane.

Here's another thread on the subject (these people were driving to Vancouver): https://community.ricksteves.com/travel-forum/transportation/any-americans-choose-to-use-vancouver-bc-airport-as-flight-departure-and-re-entry

It will be fine. We live in the USA and sometimes have flown to Europe from Canada and it is no different really from traveling to Europe from the USA except that you have to pass into Canada and border control but your passport is all you'll need. I hope you have a great trip.

When driving into or flying to Canada, does one need to block off time to go through Canadian Customs?

When driving from Seattle to Vancouver, the line is the main issue at the border crossing. Questioning is usually brief for US citizens.

Should one fly to Canada on a separate ticket to the ticket that takes them to Europe, would they not need to go through Canadian Customs?

Anyone arriving at Montreal (Trudeau) airport from a foreign country has to go through Immigration/Passport Control.

I am pretty sure my post above is incorrect but it won't delete for me. Sorry. Pay me no mind.

In addition, anyone going to Montreal has to go through Norma. She can be a real stickler for grammar and punctuation. Allow plenty of time. :)

Norma,
Thank you for answering my question even if you think your first post is incorrect.

You can now connect at Montreal without going through Canadian customs if flying on to the USA, providing your luggage is checked through and you have a boarding pass for your onward flight. This did not used to be the case. You will though need to go through USA preclearance.

Connecting in Montreal to an international flight from the USA requires you to go through 'international connections customs' if you have everything checked through and a boarding pass, otherwise you need to enter Canada then depart.

Yes, right after Passport Control you will find me at the Grammar Police Desk. A short question and answer (written) test has to be undergone by everyone whose mother tongue is English. Have your documents in hand.

Thank you, Marco, for your answer.

More details about the transfer process in Montreal Airport are here: http://www.admtl.com/en/guide/connecting-flights

For Toronto, it depends on your exact flights; here's their webtool to allow you to determine the procedures: http://www.torontopearson.com/Connecting.aspx#

And here's the guide for Vancouver: http://www.yvr.ca/en/navigating-yvr/passenger-guides/connecting-through-yvr.aspx

Funny - none of them mention Norma [g].

Thanks for the information, everybody. We're going to give it a try. We booked two tickets on Air France at almost exactly half the price of flying out of Atlanta. Even when you include the driving and extra hotels, the savings amount to $700.

Be sure to stop by the Grammar Police Desk!

I'll be looking for you, Norma.