The nearly six-week-long televised trial revived long-simmering debates about how postpartum mental health in women is treated in the US, and how the country's legal system addresses cases when mothers kill their children.
No one disputed that Clancy, 36, strangled her three children to death in January 2023 with exercise bands in the basement of her home in the Boston suburb of Duxbury.
She then cut herself with a knife and jumped from a second-storey window in a failed attempt to end her own life that left her paralysed.
Her lawyer Kevin Reddington throughout the trial sought to convince jurors that she should be found not guilty by reason of insanity, arguing that she was a loving mother who had been in the midst of a psychotic episode when she killed five-year-old Cora, three-year-old Dawson and eight-month-old Callan on January 24, 2023.
Testimony from family members including Clancy's now ex-husband, Patrick, detailed how she had struggled with her mental health in the preceding months following the birth of her third child, repeatedly seeking treatment from healthcare providers who prescribed her a myriad of drugs.
The trial drew comparisons to the case of Andrea Yates, a Texas woman who drowned her five children in a bathtub in 2001.
After an appeals court overturned her initial murder conviction, she was found not guilty by reason of insanity in 2006.
Her lawyers had said she suffered from severe postpartum psychosis.
Prosecutors did not dispute that Clancy, a former labour and delivery nurse at Massachusetts General Hospital, had been dealing with mental health issues. But they argued that she was able to appreciate the wrongfulness of killing her children and intentionally chose to do so anyway.
They presented testimony from a psychologist who, after examining Clancy, concluded that she had planned to commit suicide and chose to kill her children because "she was convinced that they would suffer without her".
Prosecutors focused on evidence showing how shortly before the killings, she sent her husband out to pick up a food order and go to a pharmacy after mapping on her phone the time it would take for him to return home.
Patrick Clancy testified that his wife had been discharged from a psychiatric hospital less than two weeks earlier but showed no signs she planned to hurt herself or her kids in the days that followed.
She had a "normal" demeanour when he left to pick up dinner, he said.
He spoke on the phone briefly with his wife while at the pharmacy.
Upon his return, he discovered her lying on the ground in the backyard with cuts.
She told him she had tried to take her own life.
He said he asked where the children were, and she said they were in the basement.
After he called 911 and first responders arrived, Patrick Clancy said he ran to the basement and found they were dead.
Several witnesses including Patrick Clancy testified that later, while Lindsay Clancy was at a hospital, she spoke about having heard a man's voice tell her that if she did not act, she would lose her chance.
The mistrial, on the seventh day of deliberations, leaves the case unresolved and allows prosecutors to take Clancy to trial again.
Neither Clancy, her family nor the jury showed any emotion as the judge announced the mistrial.
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