I am a Canadian residing/working in the US since September 2019, prior to which I was resident/working in Canada. Having resided for less than 183 days in the US, should I be filing US taxes for year 2019 as a 'Non-Resident Alien'? And which return should I be filing first: Canadian or US?
If you entered the United States in 2019 and resided less than 183 days you do not qualify as a resident alien under the Substantial Presence Test. For the 2019 tax year you are considered non-resident alien.
As a non-resident alien you are not able to file a Form 1040 but must file Form 1040NR. This non-resident tax form is not supported in TurboTax. You can use Sprintax to file your Form 1040NR.
When filing Form 1040NR you would only be including the income that you earned in the Untied States. You would not include any income that you had earned solely in Canada.
If you are taxed in both Canada and the Untied States for the same income, you might be eligible to claim a Foreign Tax Credit for the taxes you paid to Canada. You should prepare your Canadian return first to determine the amount of income taxes you will be paying to Canada on the shared income.