"Mum Bett" was born into slavery in New York and was probably a teenager when she became the property of Colonel John Ashley in 1746. Tradition has it that John Ashley, one of the most prominent and respected men in western Massachusetts, treated his enslaved men and women relatively well. His wife, on the other hand, was described as "a shrew untamable."

In a fit of anger one day, Mrs. Ashley raised a red-hot kitchen shovel to strike another Ashley slave, Lizzie. (The historical record is not clear, but it seems likely that Lizzie was either "Mum Bett's" sister or daughter.) "Mum Bett" took the blow instead and bore a scar on her face for the rest of her life. She left the Ashley household shortly after this incident. She sought help from Theodore Sedgwick, a young lawyer in the nearby town of Stockbridge, who was known to support the anti-slavery cause. Years later he told his daughter Catharine that "Mum Bett" believed she had a case based on the newly ratified Massachusetts Constitution. When he asked how she knew of this, she is said to have replied, "By keepin' still and mindin' things."

"When she was waiting at table [in the Ashley home]," Catharine Sedgwick explained, "she heard gentlemen talking over the Bill of Rights . . and in all they said she never heard but that all people were born free and equal, and she thought long about it, and resolved she would try whether she did not come in among them." Theodore Sedgwick would later say that there was nothing"submissive or subdued" about "Mum Bet's" character. But because women had such limited legal rights, the lawyers decided to add a male slave, known simply as "Brom," as a party to the suit.

It appears that Theodore Sedgwick and other men of conscience in Berkshire County chose to make Brom and Bett v. Ashley a test case. Some of the most able members of the bar were involved on both sides of the case.

On August 21, 1781, the plaintiffs' attorneys argued before the Court of Common Pleas that "Mum Bett" and Brom were not the property of John Ashley and should be freed. The following day, the jury decided in favor of Brom and Bett. They were awarded 30 shillings in damages and Colonel Ashley was assessed almost six pounds in court costs. Ashley appealed to the Supreme Judicial Court, but in October, he changed his mind and accepted the lower court's judgment: "Mum Bett" and Brom were free.

Around the same time that "Mum Bett" and Brom won their freedom in Berkshire County, another case was working its way through the Massachusetts courts. In the spring of 1781, an enslaved man named Quok Walker had sued for his freedom in Worcester. Walker had run away. When his master found him, he beat him and locked him up for several hours. Walker brought charges for assault and battery. Jurors who heard the case left no record of their reasoning, but they decided that Quok Walker should be free.

This was not the end of the matter. The case continued for several years, involving civil and criminal charges and appeals. The constitutionality of slavery was clearly raised when the last appeal was heard in April 1783. It was during this trial that Chief Justice William Cushing told the jury: "All men are born free and equal: and . . . every subject is entitled to liberty; and to have it guarded by the laws. . . .Perpetual servitude can no longer be tolerated in our government. . . . " The jury agreed, confirming Quok Walker's right to freedom.

Back in Sheffield, John Ashley asked "Mum Bett" several times to return to his home as a paid employee. She declined, preferring to serve as the Sedgwicks' housekeeper. Theodore Sedgwick, her attorney, became a United States Senator and later a justice of the Supreme Judicial Court. "Mum Bett" chose the name Elizabeth Freeman, although most people continued to call her by her old name (also spelled "Mumbett" and "Mumbet)". In 20 years, she saved enough money to buy a plot of land, where she lived with her daughter, grandchildren, and great-grandchildren. She died in 1829 in her mid-80s and is the only non-family member buried in the Sedgwick family plot in the Stockbridge cemetery.