So if you want to file your taxes as married do you have to be legally married with a marriage license? Does a common law marriage count for filing as married?

I think it depends on where you live. Some states don't recognize common law marriages.

I live in Washington state. But income tax is a federal law so how is it looked at federally is what I am asking.

I'm not sure. Hopefully someone more experienced with tax law will come and answer your question. Sometimes it takes some time, so check back periodically.

Good Luck.

This is form IRS publication 17:
You are considered married if on the last date of the tax year
"You are living together in a common law marriage that is recognized in the state where you now live or in the state where the common law marriage began."

The issue as stated before marriage is a state issue not federal one.
So to be married, you have to meet the conditions of the state you live in,
If you live in one of the few US states that still reconise common law marriage, and have been together the years required, and meet the requirements of common law marriage, then you are married under that state law, If you meet your states law for marriage, on Dec 31, then you can file as married, if you do not meet the common law rules of your state, or if your state does not reconise common law, and you are not married under the laws of that state, you can not claim married

Also you noted Washington state, according to this web site
Common Law Marriage (http://www.ncsl.org/programs/cyf/commonlaw.htm)

And another site

Common Law Marriage (http://www.expertlaw.com/library/family_law/common_law.html)

And another site

Common Law Marriage FAQ (http://www.nolo.com/article.cfm/pg/2/objectId/709FAEE4-ABEA-4E17-BA34836388313A3C/catId/1f8cd39f-a86d-4a11-a90eb147c32edf2c/118/FAQ/)

Washington State does not reconise common law marriage any longer, and in fact they do not allow those to be grandfathered into common law.
So since common law does not seem to be available in your state, then there would have to be a formal marriage license from the state to become legally married.

** not picking, I personally don't care, law is just my area that I actually answer more in the legal areas than any other, it has been my area of expertise .
And on the marriage issues, since I am over a large group of ministers in almost every state of the US, Canada and other nations, our office here has to be up on most of the marriage laws of each state to advise our Priests what they have to do. For example in common law states, if they are already have a common law marriage, they can merely bless the union. And we often, I do many every year, where I merely bless their union, I do this for seniors all the time, where it hurts them to get married, ** they lose social security benefits of their spouses who passed away. So often we bless their unions so they are "married" in Gods eyes, and we would have blessed your union and to the church you would be married, the issue is that to the church you are married, just not to the state since you have not meet their legal requirements.

Agree with all above, with one additional note.

Some states recognize same-sex marriages as legal marriages.

The IRS does NOT, under any circumstance, recognize same-sex marriages as a legal marriage for tax purposes.

That may change in the future, especially if either Hillary or Barack get elected, but, right now, if your marriage is of the same-sex variety, you CANNOT file Married Filing Jointly or Married Filing Separately.