A lot changed in that span. International pressure mounted against Peru's secretive military trials that heard terror cases, the government in power in Peru at the time of Berenson's conviction was discredited, and the wave of terror that grasped Peru in the 1980s and 1990s receded.
Berenson was arrested in November 1995 and accused of conspiring with terrorists and helping them rent safe houses and gather intelligence for attacks.
Her sentence now complete, Peruvian judicial officials told CNN she will be "expelled" from the country as soon as the required paperwork is done.
Berenson's lawyer confirmed that her sentence was completed Sunday and that "she can leave" after she signs some paperwork.
Now 46, Berenson will return to the United States having at one point been sentenced to life in prison, and, also, as the mother of a 6-year-old boy.
Berenson served the last five years of her sentence on parole and was granted permission in 2011 to visit her family in New York for the December holidays. That permission was controversial because of concerns by some in Peru that Berenson would not return to finish her sentence. But she did.
Berenson told CNN en Español on Tuesday she would not grant interviews until she arrives in New York.
Berenson was sentenced by a military court to life in prison in January 1996 for collaborating with the Tupac Amaru Revolutionary Movement to attack the nation's Congress and overthrow the government. The attack never took place.
The New York Times bashed the military tribunal in a 1996 editorial, noting that Berenson's attorneys were not allowed to cross-examine witnesses or challenge evidence.
The efforts to get the U.S. government to take a stand on behalf of Bernenson included a letter to President Bill Clinton, signed by 176 members of the U.S. House of Representatives.
In 2000, her military court conviction and sentence were overturned, and she was tried in a civilian court, which also found her guilty and sentenced her to 20 years, with credit for time served.
She was released from Lima's Chorrillos Penitentiary in May 2010 after a Peruvian court granted her parole.
Before Chorrillos, Berenson was held in other prisons, some located more than 10,000 feet above sea level, where her health suffered.
In May 2009, while in prison, Berenson gave birth to a son fathered by Anibal Augusto Apari Sanchez, her estranged husband who is also her attorney. The boy lived with her in prison until her release.