ALL the income of an Illinois resident is taxable by Illinois, regardless of where it was earned.
A Illinois non-resident's earned income (income from work) is not taxable by Illinois if none of the work was actually (physically) performed in Illinois.
Assuming the non-resident taxpayer had no other Illinois-source income (such as rental income from a property located in Illinois), the taxpayer would not be required to file an Illinois tax return.
However, if the non-resident taxpayer's employer mistakenly withheld Illinois taxes, the taxpayer would have to file a non-resident Illinois return, showing zero Illinois income, in order to have those withheld taxes refunded. But the taxpayer would not be required to do so.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
