Sorry that it took me a while to get to this.

As /u/mormengil mentioned, Deborah Sampson is the only woman, to my knowledge, to disguise herself as a man and maintain that disguise long enough to see combat. It should be noted that Sampson served with distinction: She was admitted to the Light Infantry, the chosen men of the American Army selected for their physical ability and trustworthiness. This is why she saw repeated action against loyalist militia in the Lower Hudson Valley.

Margaret Hayes, still remembered to schoolchildren as "Molly Pitcher," was one of several women who served on a gun crew in action. While Hayes had the fortune to avoid injury (a cannon ball passed between her legs, ripping her petticoats. She supposedly quipped something to the effect "I'm glad it didn't bounce any higher, and carry away something more precious."), the inexplicably less famous Margaret Corbin was less lucky. When her husband was killed manning his gun during the British assault on Fort Washington (now subsumed by the Bronx, New York City), Corbin took his place. She was seriously wounded by British canister shot, suffering debilitating and disfiguring injuries. While women whose husbands died were usually turned out of the army if they didn't quickly remarry, Corbin was allowed to stay on, drawing half-pay (!) and rations as a member of the Corps of Invalids.

A frontier militia was actually the least likely place you'd see a woman in combat. Militia units, even by period standards, were pretty intimately connected, since they were raised locally (by county, generally). They tended to be composed of more established members or the community: The young, desperate, or unconnected tended to join the Continental Army. As such, it would be difficult to join a militia unit under an assumed identity, since there wasn't really space for strangers to join. In the regular army, recruiting areas were wider, particularly by the end of the war, when Sampson joined up. Being that the army recruited heavily among recent immigrants and transients, it would be easier to slip in while disguised.

The best accredited woman who fought in the American Revolutionary War was Deborah Samson, who served in a Massachusetts Regiment for 3 years disguised as a man under the pseudonym of Robert Shirtliffe.

She was wounded twice, but survived the war, married, and had 3 children.

There were other women who fought in individual engagements.

This source: http://userpages.aug.com/captbarb/femvets.html , gives brief accounts of some of them.