Civilian Pay Earned by Active Duty Military
When an active duty service member also has civilian/nonmilitary wages
The Service Members Civil Relief Act of 2003 continued to provide that active duty military members may only be taxed on their military pay by their state of legal residence (often the same as their home of record.)
If you're not sure of your military state of legal residence (SLR), check out Filing State Income Taxes When You Are in the Military.
However, if you are an active duty service member and also work a nonmilitary/civilian job or are freelancing, you will probably have to file taxes as a nonresident in the state you are physically working in.
If you have to file a state tax return for your state of legal residence, most of the time you can get a credit for the taxes paid on your nonresident return.
Here's an example.
John joined the military in Georgia, and Georgia is his state of legal residence. However, he's currently stationed in Virginia. On the weekends, he works at the local hardware store.
John will have to file a Virginia nonresident tax return and will only be taxed on his nonmilitary wages. He'll also file a resident state return with Georgia. The Georgia return will include both his military and nonmilitary wages and all other taxable income. Georgia allows a credit for the Virginia taxes paid on the Georgia state tax return.
Spouse of an active duty service member
The rules for what state the spouse of a military service member needs to file in are not the same as for active duty military. For more information, see Military Spouses and State Taxes.