A jet-setting socialite who once consorted with royals, presidents and billionaires, Ghislaine Maxwell, was sentenced to 20 years in prison Tuesday for helping the financier Jeffrey Epstein sexually abuse underage girls.

The sentence was the punctuation mark on a trial that explored the sordid rituals of a predator power couple who courted the rich and famous as they lured vulnerable girls as young as 14, and then exploited them.

Prosecutors said Epstein, who killed himself in 2019 while awaiting trial, sexually abused children hundreds of times over more than a decade, and couldn’t have done so without the help of Maxwell, his longtime companion and onetime girlfriend who they said sometimes also participated in the abuse. In December, a jury convicted Maxwell of sex trafficking, transporting a minor to participate in illegal sex acts, and two conspiracy charges.

U.S. District Judge Alison J. Nathan, who also imposed a $750,000 fine, said “a very significant sentence is necessary” and that she wanted to send an “unmistakable message” that these kinds of crimes would be punished. Prosecutors had asked the judge to give her 30 to 55 years in prison, while Maxwell’s defense sought a sentence of just five years.

Maxwell, wearing a blue prison uniform and a white mask to conform with coronavirus rules, looked to one side as the sentence was announced, but otherwise did not react.

“We will continue to live with the harm she caused us,” one of the four accusers who testified against Maxwell at trial, Annie Farmer, said inside the courtroom before the sentencing.

When she had a chance to speak, Maxwell said she empathized with the survivors and that it was her “greatest regret of my life that I ever met Jeffrey Epstein.” Maxwell called him “a manipulative, cunning, and controlling man who lived a profoundly compartmentalized life,” echoing her defense attorneys’ assertions that Epstein was the true mastermind.

Maxwell, who denies abusing anyone, said she hoped that her conviction and her “unusual incarceration” bring some “measure of peace and finality.”

Judge Nathan refused to let Maxwell escape culpability, making clear that Maxwell was being punished for her own actions, not Epstein’s. She called the crimes “heinous and predatory” and said Maxwell as a sophisticated adult woman provided the veneer of safety as she “normalized” sexual abuse through her involvement, encouragement and instruction.

Several survivors described their sexual abuse, including Ms. Farmer, who said her sister and herself tried to go public with their stories about Epstein and Maxwell two decades ago, only to be shut down by the powerful couple through threats and influence with authorities.

Inside the crowded courtroom, three of Maxwell’s siblings sat in a row behind her. Most of the others in attendance were members of the media.

Assistant U.S. Attorney Alison Moe recounted how Maxwell subjected girls to “horrifying nightmares” by taking them to Epstein.

“They were partners in crime together and they molested these kids together,” she said, calling Maxwell “a person who was indifferent to the suffering of other human beings.”

Epstein and Maxwell’s associations with some of the world’s most famous people were not a prominent part of the trial, but mentions of friends like Presidents  Clinton and Trump and Prince Andrew of Britain showed how the pair exploited their connections to impress their prey.

Over the past 17 years, scores of women have accused Epstein of abuse them, with many describing Maxwell as the madam who recruited them. The trial, though, revolved around allegations from only a handful of those women.

Four testified that they were abused as teens in the 1990s and early 2000s at Epstein’s mansions in Florida, New York, New Mexico, and the Virgin Islands.

Three were identified in court only by their first names or pseudonyms to protect their privacy: Jane, a television actress; Kate, an ex-model from the U.K.; and Carolyn, now a mother recovering from drug addiction. The fourth was Ms. Farmer, the sole accuser to identify herself in court by her real name, after speaking out publicly.

They described how Maxwell charmed them with conversation and gifts and promises that Epstein could use his wealth and connections to help fulfill their dreams.

Then, they testified, she led them to give massages to Epstein that turned sexual and played it off as normal.

Carolyn testified that she was one of several underprivileged teens who lived near Epstein’s Florida home in the early 2000s and took up an offer to massage him in exchange for $100 bills in what prosecutors described as “a pyramid of abuse.”

Maxwell made all the arrangements, Carolyn told the jury, even though she knew the girl was only 14 at the time.

The allegations against Epstein first surfaced publicly in 2005. He pleaded guilty to sex charges in Florida and served 13 months in jail, much of it in a work-release program as part of a deal criticized as lenient. Afterward, he was required to register as a sex offender.

In the years that followed, many women sued Epstein over alleged abuse. One, Virginia Giuffre, claimed that Epstein and Maxwell had also pressured her into sexual trysts with other powerful men, including Prince Andrew. All of those men denied the allegations and Ms. Giuffre ultimately settled a lawsuit against Andrew out of court.

Federal prosecutors at New York revived the case against Epstein after stories by the Miami Herald in 2018 brought new attention to his crimes. He was arrested in 2019, but killed himself a month later.

Eleven months after his death, Maxwell was arrested at a New Hampshire estate. An American, British and French citizen, she has remained in a federal jail at New York City since then as her lawyers repeatedly criticize her treatment, saying she was even unjustly placed under suicide watch days before sentencing. Prosecutors say the claims about the jail are exaggerated and that Maxwell has been treated better than other prisoners.

Her lawyers also fought to have her conviction tossed on the grounds of juror misconduct. Days after the verdict, one juror gave media interviews in which he disclosed he had been sexually abused as a child — something he hadn’t told the court during jury selection. Maxwell’s lawyers said she deserved a new trial. A judge disagreed.

At least eight women submitted letters to the judge, describing the sexual abuse they said they endured for having met Maxwell and Epstein. Six of Maxwell’s seven living siblings wrote to plead for leniency. Maxwell’s fellow inmate also submitted a letter describing how Maxwell has helped to educate other inmates over the last two years.

Her eldest siblings, Anne Holve and Philip Maxwell, wrote that her relationship with Epstein began soon after the 1991 death of their father, the British newspaper magnate Robert Maxwell.

They said Robert Maxwell had subjected his daughter to “frequent rapid mood swings, huge rages, and rejections.”

“This led her to becoming very vulnerable to abusive and powerful men who would be able to take advantage of her innate good nature,” they wrote.

Before her fate was announced, Maxwell looked down and scribbled on a notepad as an accuser whose allegations weren’t included in this trial, Sarah Ransome, spoke of the lasting harm to her life, gazing directly at Maxwell several times. Ms. Ransome, who twice tried to die by suicide, finally drew a look from Maxwell when she said: “You broke me in unfathomable ways but you did not break my spirit.”