If you do not live in a common law marriage state then you have to file as Single on each tax return. Or one of you can file as Head of Household. You cannot file as married if you are not legally married.
Here are the places that recognize common-law marriage: Colorado, Iowa, Kansas, Montana, New Hampshire (for inheritance purposes only), Oklahoma, Rhode Island, South Carolina, Texas, Utah and the District of Columbia.
No, you can never file a joint return unless you are legally married.
If you live (or lived) in a state that recognizes common-law marriage, you may be legally married even if you did not have a ceremony. Those states are Colorado, Iowa, Kansas, Montana, New Hampshire, South Carolina, Texas and Utah. Exact requirements vary by state but the general requirements are usually,
- Your marriage would not be illegal (such as because you are married to someone else or because you are closely related)
- You live together and share your lives as if you were married
- You present yourself to other people as being married.
Be aware that once you have a common law marriage in a state that recognizes it, then you are legally married in every state, now and forever, in sickness and in health, till death do you part, unless you go to court with real lawyers and a real judge and get a real divorce. Common law marriage can't be turned on and off like a light switch, or for the convenience of your tax returns. If you want to file a tax return as being common law married, then you are married regarding everything else in your life where it might matter (health insurance, banks and finance, joint debts, and so on).
Deciding that you have a common law marriage is not simple, and you may want legal advice. If you have a common law marriage then you are legally married and you must file a joint tax return, and you may have to go back and file amended tax returns for previous years if you were married but filed singly.