Depends on your State of residency. Some states do recognize common-law marriages. Check your state's local laws.
It depends on what you mean by “lived with my partner."
If you are just “living together” without any type of formally recognized arrangement then you can never file together.
For federal tax, you can’t file a joint return unless you are married. The IRS does not recognize registered domestic partnerships, civil unions, or other similar formal relationships that are not marriages under state law.
Some states do recognize common law marriages (as awesome community member @Butty-012249 says), along with other arrangements.
Learn more at:
Q. Do I file jointly if I’ve lived with my partner the entire year but we’re not married?
A. No. You must be legally married.
You may be able to file as Married Filing Jointly if you live in a state that recognizes common law marriage. Currently, the following jurisdictions recognize common law marriage:
Colorado
District of Columbia
Iowa
Kansas
Montana
New Hampshire
Oklahoma
Rhode Island
South Carolina
Texas
Utah
Alabama (2017), Georgia(1997), Idaho(1996), Ohio(Oct. 1991), and Pennsylvania(2005) are grandfathered for the marriages before the year indicated. Living together in a common law state is usually insufficient - you need to hold yourself out as married including owning property together, having joint bank accounts, etc. To find out your state’s rules see:
http://family.findlaw.com/marriage/common-law-marriage-states.html