The questions might arise about tax relief USA for citizens who live abroad. If you are a U.S. citizen, it means that your world income is taxable under U.S. laws. In other words what you earn abroad is also taxable in the USA. Obviously, this will apply if you work in Canada as well. So, you need to check your filing requirements the way they are based on U.S. laws.
But keep in mind that this does not mean that Canadian tax laws will be not applicable to you. The best is to check in this matter rather than remaining oblivious that you have paid your U.S. tax. You might be obligated to file a tax return with Canadian authorities and pay taxes in Canada as well. In that situation, though, you will be entitled to tax relief USA from the IRS, which will either give you credit for the tax paid in Canada or enable you to exclude part or all of your income that you earned in Canada.
You need to bear in mind that benefits expected from the Canada Pension Plan, as well as the Old Age Security scheme, are not taxable in the U.S. You may be a U.S. citizen or green card holder in the U.S., as long as you are resident of Canada. This tax relief originates from a tax treaty between Canada and the USA, which provides that these benefits will be taxed exclusively on the basis of residence. Equally, these benefits become taxable if you get them while you are residing in the U.S.
Such Canadian social security benefits are treated equally with comparable U.S. benefits for the reason of U.S. tax. But, if any of these benefits is not taxable in the hands of a Canadian citizen under the law, the same benefit cannot be taxed in the U.S., as added to providing you with tax relief.