Where would a US citizen pay taxes if he/she would be employed by a Canadian employer working mainly the US?

Answer 1

To the IRS. You may have some issues with the Canadians depending on how the income is reported but the IRS will want the US tax.

Answer 2

Income is generally subject to tax in the country where an individual performs the work. Assuming that this U.S. Citizen is not a permanent resident of Canada, the U.S. would get the tax on the income and Canada should not have claim on the income.

But in some countries like Canada income is also subject to tax in the country where the individual is a permanent resident. When that happens, the country of residence allows an offset (tax credit) of the tax paid to the country where the work is performed.

Answer 3

"working mainly the US"

Where is the rest of the work being done? Canada could be interested in any days of work done in Canada (from publication 597):

Income U.S. residents receive for the performance of dependent personal services in Canada (except as public entertainers) is exempt from Canadian tax if it is not more than $10,000 in Canadian currency for the year. If it is more than $10,000 for the year, it is exempt only if:

The residents are present in Canada for no more than 183 days during the calendar year, and

The income is not paid by a Canadian resident employer or by a permanent establishment or fixed base of an employer in Canada.

So...if the portion of the time spent in Canada equates to $10,000 or more, you will have a Canadian tax return to do.

Answer 4

Your employer's location is irrelevant. You're a US citizen working in the US. You and your employer are both subject to US tax laws. Your employer must withhold taxes from your wages and you file the exact same tax returns you always have.