At the end of the 18th century in the USA, the majority of the population was still engaged in farming. All the jobs on a farm are available to women, and not uncommonly performed by women, so the statistically most likely occupation was farmworker.
The second most likely occupation is probably trades. I will assert without proof that there were very few trades where there wasn't at least one example of a woman tradesman. While many of them were widows, (not specifically excluded by your question), some learned the trade through other means.
This question should have been closed as trivial, because if you spend 30s on google, you'll find multiple examples. If you enter "18th century female american" and any profession, you'll come up with dozens of examples. The following are just a quick sample I came up with in 15 minutes. (I prudently did not ask my professional historian girlfriend, because this question would have triggered a tirade that probably would have reached through me to the internet. She has rather strong opinions on the historiography of women - we've accepted a myth that they were all prostitutes or helpless, waiting for rescue by a man, and that men devoted a significant fraction of their waking hours to finding new woman to oppress in new ways. This simply isn't supported by historical facts.)
If I spent more time, I'd have a longer list.