The man and woman, who cannot be legally named, lived on a NSW South West Slopes property neighbouring where the young girl lived on and off with her extended family.
In late 2000, the teenager moved in with the couple and formed a sexual relationship with the then 40-year-old man, becoming pregnant and having a child.
The 19-year-old was last seen by eyewitnesses at a flat she was renting in a nearby town on June 2, 2002, and a police investigation was launched into her disappearance.
In front of a packed courtroom, Justice Julia Lonergan on Monday acquitted the couple of the girl's murder after a 28-day judge-alone trial in the Supreme Court.
One man swiftly departed the courtroom after the verdict, letting out a scream in the building's corridors.
The judge found prosecutors failed to prove the couple jointly agreed to murder the young woman to obtain her child.
There was sufficient evidence to show the 19-year-old died in 2002, Justice Lonergan said in her 107-page judgment.
However, she said this was after the couple said they had dropped her off at Campbelltown train station to go and see her dying father on June 5 that year but before June 19 when a Centrelink payment was deposited into her bank account but not touched.
After that time, she neither contacted her family nor bought medication for the life-threatening epilepsy she had been diagnosed with as a child.
"I have decided that the version of events provided by the accused may be true," Justice Lonergan told the court.
"In those circumstances, I must acquit."
The young woman's parenting was not undermined by the man and woman who did not prevent her from seeing or talking to immediate family members despite their prior neglect and abuse, the judge said.
"She was physically attacked, abused and made to feel unsafe or unwanted, or both, by members of her own family," Justice Lonergan said.
"How terrible that must have felt for a vulnerable, loving young woman carrying a life-threatening illness, the deep scars of a jagged, abusive childhood and cognitive and processing difficulties."
The young woman had bounced back and forth between people and places looking for love and solace.
"She never found it. She was still looking for it when she disappeared," the judge said.
Medical evidence presented the woman as someone with the mental capacity of a 12- or 13-year-old who did not understand the consequences of her actions.
"(The woman) seems to have said strange and silly things for effect or attention," the judge noted.
While healthcare staff dealing with the teen kept things objective and professional, the involvement of a childhood nurse in January 2002 meant that "fact became mixed with rumour, exaggeration and speculation," she said.
Police were criticised for the handling of their more than 20-year-long investigation, with the judge saying officers were too focused on disproving the couple's version of events.
The detective sergeant involved in 2002 "was not prepared to let evidence get in the way of his own belief as to whom he thought was responsible", she wrote.
There were no incriminating conversations captured by listening devices in the couple's home and no remains or other evidence had been uncovered during a search of their property.
Prosecutors' reliance at trial on a "red herring" witness who said he could disprove the couple's claims they had dropped the woman at Campbelltown was surprising because police had already "catastrophically undermined" and disproved his allegations, the judge said.
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