Checkpoints and Borders Policy Debate - Which passport to use when entering Canada?

My partner holds both US and UK passports whilst I hold a UK passport only.

We are based in the UK and will shortly be flying from MXP to YYZ via LHR, and after a few days in Canada we will be taking a rental car from Toronto to Cleveland to visit my partner's folks. We will then be returning to Toronto by car before flying back to Europe.

Clearly she will use her US passport to enter the US from Canada and vice versa, but it would make sense (I think) for her to travel from Europe to Canada and back on her UK passport.

The question is, will there be any potential problems if she enters YYZ on her UK passport and then leaves Canada at a border crossing into the US on her US passport, or would it be better for her to use her US passport exclusively throughout the trip?

Any suggestions from those 'in the know' will be greatly appreciated.

Entering the US she must use her US passport.

Returning home to the UK would obviously favor her UK passport.

All other crossings are up to her discretion, however I would use the US passport since that does not require a stamp, as opposed to the UK passport which does.

There should be no issue using different passports at different entry points. As has been mentioned she should use her US passport to enter the US. My suggestion would be to use the UK passport to enter Canada since you are currently based in the UK and I assume that is the address you will put down on your entry card.

I currently live in the US and hold a UK passport. I also hold a US passport having become a Citizen here a couple of year ago. I frequently travel to Canada to visit family and I always found that entering Canada with the UK passport was more of a hassle that the US passport. Reason was that Canadian Immigration would want to know why a UK citizen was living in the US and how I came to be here etc. One time they even asked to see my green card when I was a GC holder. Now I have switched to using the US passport to enter Canada and find it invites far less questions.

On that basis showing the UK passport may make more sense.

Entering the US she must use her US passport.

Returning home to the UK would obviously favor her UK passport.

All other crossings are up to her discretion, however I would use the US passport since that does not require a stamp, as opposed to the UK passport which does.

Entering the US she ought to use her US passport. That, however, does not mean a US citizen must use a US passport to enter the US. US citizens have a fundamental right to be admitted into the US with or without a US passport/passport card. Presenting a US passport/passport card will generally not contribute to a delay in a US citizen being (re-)cleared, while failure to present a US passport at a US POE may more often contribute to some delay.

Canada won't care if a US citizen uses a valid UK passport for entry into Canada. Canadian BSA/passport control personnel will stamp US passports on entry too just like they would with UK passports in the main.

That said, I'd suggest the OP's partner default to maximum use of the US passport when it comes to Europe to North America trips.