…or any US resident or citizen living anywhere else in the world, income tax returns need to be filed in the US using the customary Form 1040. This can be filed either separately or jointly (if married).
If you are employed by a US company in Canada, then you fall under the Canada US Income Tax Convention. This means that your employment income is exempt from taxation by Canada unless it is above $10,000 per year. If higher than $10,000, your income is exempt if:
The individual resided in Canada for less than 183 days in any Calendar year
The exemption is not borne by a Canadian citizen or a Canadian resident employer with a fixed base in Canada
If you are self employed US citizen/resident in Canada, aside from filing Form 1040 and Schedule C in the US to declare your worldwide earnings there are other things involved. Based on your current residence, you are required to either pay the US’s self-employment tax (15.3% for incomes up to $118,500) or Canadian Pension Plan (CPP) premiums to cover your social security taxes.
Whether you are considered employed or not in Canada is dependent on whether your primary work place falls under the definition of “Permanent Establishment” as per the Canada-US Tax Treaty.