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The one about having an ax murderer in the family turns out to be no joke for Cynthia Nixon.

The "Sex And The City" star discovered that her great-great-great grandmother, Martha Curnutt, killed her abusive husband, Noah Casto, with an ax in 1840s Missouri.

The 48-year-old actress learned the shocking connection on Wednesday's premiere of the celebrity genealogy show "Who Do You Think You Are?"

While looking into Nixon's father's family history, Ancestry.com research manager and family historian Jennifer Utley uncovered a newspaper article from 1843.

Nixon teared up while reading aloud the account of Curnutt, whose married name had been Casto.

"[Noah Casto] had been in the habit of treating his wife in a manner too brutal and too shocking to think of," she read. "On the morning of the day mentioned he told his wife to get up and get breakfast for himself and her two children, and then to commence saying her prayers, for she should die, he swore, before sunset."

The article continued, "She got up and made a fire and returned to the room where her unnatural husband slept. He was lying on his back in a sound sleep. She took the ax with which she had been chopping wood and with one blow sunk it deep into his head, just through the eyes."

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Curnutt was found guilty of first-degree manslaughter and sentenced to five years in the Missouri State Penitentiary. At the time, she was the only female prisoner out of 800 inmates.

"It is awful to think of what Martha endured," Nixon said. "I certainly wouldn't call [the murder] a happy ending, by any stretch of the imagination, but certainly I think a better ending than if Noah had killed Martha and maybe killed her children, too."

While visiting the prison where her relative was held, Nixon learned that Curnutt gave birth to a daughter, Sarah, while behind bars, though the father's identity was unknown.

It was then that many prominent people in the state, including future governor and congressman, Willard Preble Hall, and a former governor, Lilburn Boggs, petitioned Missouri Gov. John Edwards to pardon her.

In December, 1844, Curnutt was pardoned after serving less than two years of her sentence. Upon her release, she returned to using her maiden name, Curnutt.

Nixon held the actual petition and pardon at the Missouri State Archives in Jefferson City and paid a visit to Curnutt's grave in Leasburg, Missouri, where she placed flowers in her memory.

"I admire how she must have wanted to give up so many times, and how she kept going and how she didn't accept things," Nixon said on the show. "I'm sure we'll make many jokes about it in the years to come, about the ax murderess in our family, but I think we will remain in awe of her."