Durst, 78, was convicted in Los Angeles Superior Court last month of first-degree murder for shooting Susan Berman point-blank in the back of the head at her home in December 2000. Durst silenced Berman to prevent her from incriminating him in the reopened investigation of his wife's 1982 disappearance in New York, prosecutors said.
Berman provided a phony alibi for Durst when Kathie Durst vanished, prosecutors said.
Prosecutors also presented evidence that he intentionally killed a neighbor in Galveston, Texas, in 2001, though he had been acquitted of murder in that case after testifying that he shot the man in self-defense.
Durst is the grandson of Joseph Durst, who founded the Durst Organization, one of Manhattan's largest commercial real estate firms. His father, Seymour, took the reins of the company and later handed control of it to a younger brother, Douglas.
Robert Durst settled his share of the family fortune and was estimated by prosecutors to have $100 million.
Durst's lawyer, Dick DeGuerin, said they plan to appeal.
Durst, who has numerous medical issues, on Thursday was rolled into the courtroom in a wheelchair wearing brown jail scrubs. His eyes were wide open and he had a catatonic stare.
Several of the jurors returned to the courtroom to witness the sentencing and were seated in the jury box.
The trial began in March 2020 and was adjourned for 14 months as the coronavirus pandemic swept the U.S. and courts were closed. It resumed in May with the jury that reached its verdict September 17.
Berman, the daughter of a Las Vegas mobster, was Durst's longtime confidante who was preparing to tell police she provided a phony alibi for him after his wife vanished in New York in 1982.
Kathie Durst has never been found. Robert Durst has never been charged with a crime related to her disappearance.
Durst testified in the Los Angeles case, denying he killed his wife or Berman, but he said he'd lie if he had done so.
Kathie McCormack Durst's family had hoped to present statements to the court Thursday about their loss, but prosecutors denied the request, according to emails sent to their lawyer.
Attorney Robert Abrams said the McCormack family was disappointed, and he was outraged. He said family members would not attend the sentencing.
"The family is not going to go travel 3,000 miles to be a prop in some Hollywood production and sit there and not be able to make their victim impact statements," Abrams said. "This is not some movie where it's gross spectacle. This is their lives, and they've suffered for 40 years."
A spokesman for the district attorney's office did not return a message seeking comment.
Loyola Law School Professor Laurie Levenson said it was questionable whether Kathie Durst was a victim in the Los Angeles case because Robert Durst wasn't charged with her killing. Allowing her siblings to speak at sentencing would create another issue for an appeal.
Defense lawyers argued in court papers that Durst deserved a new trial, saying there was insufficient evidence to prove the case and listing 15 ways the judge erred.
Judge Mark Windham rejected the motion and said the prosecution had overwhelming evidence and proved Durst's guilt beyond a reasonable doubt in at least five ways, including several admissions Durst made. He rejected all the arguments that he had ruled incorrectly.
"You said the court erred so many times it made me feel self-conscious," Windham said.
The motion was mainly to preserve issues for appeal and likely to fail, Levenson said, though some of their arguments could get a higher court's attention.
The defense said Windham should have declared a mistrial when the jury was sent home and the case adjourned. They said the length of the delay was unprecedented.
Many of the other issues revolve around admission of evidence, such as allowing prosecutors to present testimony that Durst killed a neighbor in Galveston, Texas, in 2001 and chopped up his body and tossed the parts out to sea. Durst was acquitted of murder after testifying that Morris Black pulled a gun on him and was killed in a struggle for the weapon.
They also objected to testimony and exhibits that showed when Durst was arrested in New Orleans in 2015, he had a mask, fake identification, drugs, $40,000 and a loaded handgun in his hotel room.
In a moment off camera but with a live microphone, Durst muttered, "Killed them all, of course."
Deputy District Attorney John Lewin was asked by a reporter after the conviction what he had to say to Durst.
Lewin, who was aggressive and frequently scrapped with defense lawyer Dick DeGuerin over the six years it took from arrest to conviction, initially said he didn't have anything to say to the man he had pursued for years.
Then he called Durst a "narcissistic psychopath" who "killed his wife and ... had to keep killing to cover it up."
"He's 78 years old. He's been walking around for a long time," Lewin said. "He had a lot more of a life, you know, Kathie didn't make 30. On balance, considering what he's done, he got a lot more of a life than he was entitled to."