In 1781, Mum Bett successfully sued her enslaver, John Ashley, in a Massachusetts court, arguing that slavery was incompatible with the state's new constitution which declared all men (and implicitly women) to be born free and equal. The judge ruled in her favor, setting a precedent that would eventually lead to the abolition of slavery in Massachusetts.
This was a monumental achievement, not only for Elizabeth Freeman and her daughter but for the fight against slavery in America.