Moxley had stoically endured countless legal twists and turns, including many involving a prime suspect in the killing, Kennedy cousin Michael Skakel, who was convicted of the crime and later freed after his murder conviction was vacated.
Moxley's son John told The Associated Press on Thursday that his mother died on Tuesday at her home in Summit, New Jersey, of complications from flu and possibly pneumonia.
Martha Moxley was beaten to death with a golf club. (AP PHOTO)
"Through all the ups and downs, she was the most glass-half-full person I've ever known," Moxley said.
He described his mother as a "crusader" for justice on behalf of his 15-year-old sister Martha, but "never vindictive".
The elder Moxley, who had been living in New Jersey for more than 20 years, was devoted to finding Martha's killer and keeping the case in the public eye.
The teen was beaten to death on October 30, 1975, with a golf club.
Her battered body was found the next day under a tree on her family's estate in the wealthy Belle Haven section of Greenwich, across the street from the Skakel family's home.
The shocking murder, which went unsolved for decades, became a sensation and the subject of several books, a film and a documentary series.
Robert F Kennedy Jr, a staunch defender of his cousin, released a book in 2016 about the case saying Skakel — the nephew of Kennedy's mother, Ethel — had been framed.
Skakel was arrested in 2000, convicted of murder in 2002 and sentenced to 20 years to life in prison.
Several appeals followed. After serving more than 11 years in prison, he was freed in 2013 on $US1.2 million ($A1.9 million) bail after a lower court judge overturned his conviction, saying his trial lawyer failed to adequately represent him.
The state Supreme Court reinstated the conviction in a 4-3 ruling in 2016.
But the justice who wrote the decision retired soon afterward and a new justice sided with Skakel in a highly unusual 4-3 opinion in 2018 that overturned the conviction.
The U. Supreme Court declined to hear the state's appeal in 2019 and a second trial was not pursued, in part because many of the potential witnesses had since died and there wasn't enough evidence for a retrial.
Skakel's lawyer, Stephan Seeger, called it a "day for justice" at the time and that his client was innocent of the crime.
Michael Skakel servied more than 11 years in prison for the murder. (AP PHOTO)
John Moxley told the AP in 2020 that he and his mother still believed Skakel killed Martha, but they were at peace with the decision not to seek a second trial. On Thursday, he said they ultimately did get justice for his sister.
"It was just incredible how much support we got from people that we knew and people that we didn't know," he said.
"My mother was always grateful and chose to focus on the communal spirit, if you will, rather than the law. My mother always said she believed in angels and the angels really came out to support her, to support us."
Known for her kind and co-operative demeanour, Moxley was asked by police departments in other states to meet with family members of murder victims who were uncomfortable speaking with police, John Moxley said. His mother also spoke at police and victims' advocacy conferences throughout the years.
"She was able to take that spirit of support and knowing how important it was to share that with others. And that gave her a lot of joy, knowing that she could help somebody else," he said.
"Somebody one time she was tough. She said, 'I'm not tough. I'm just strong.'"
