All US citizens and permanent residents must file federal income tax returns if they meet the IRS filing threshold. The amount of this threshold will vary depending on factors such as age, filing status, and type of income (i.e., income from employment or self-employment). For example, a single individual under the age of 65 would be required to file a 2019 US federal tax return if their gross income exceeded $12,200. If the earnings came from self-employment, this same person would need to file a US federal tax return if their net earnings exceeded $400.
When they are working outside the United States, most US citizens and permanent residents (i.e., green card holders) will be required to file income tax returns in both their Host country and in the United States. Filing two sets of returns can be a headache for the taxpayer, but it does not necessarily mean that they will be taxed twice on the income earned while working abroad.
It may seem counterintuitive to a US citizen or permanent resident (i.e., a green card holder) who has just taken a new international job, that most will still be required to file US federal income tax returns after relocating. In addition to filing income tax returns, mobile employees may also have other filing obligations including estate or gift tax returns, estimated tax payments, and foreign bank account reports.
If you have employees working outside the United States who are US citizens or permanent residents (i.e., a green card holder), these individuals will need to continue filing US tax returns to declare all of the income they earn in both the United States and their Host country. This requirement does not change if they are employed or paid from a non-US employer. Additionally, most of the tax rules that apply to taxpayers living in the United States will also apply to US persons operating overseas. The result may be that an overseas employee will be subject to tax in both the US and other jurisdictions.