Two months before she went missing, Kathie Durst received a call from her husband’s mistress.

On the other end of the phone was Prudence Farrow — the sister of Mia Farrow, a yoga instructor and the muse behind the Beatles song “Dear Prudence.”

She was also married and lived in a Durst-family-owned brownstone on West 43rd Street.

“Prudence wanted Kathie to give Bob up,” Eleanor Schwank, a college friend of Kathie’s, told The Post in June 1982. She wanted him all to herself, friends said.

Prudence Farrow Bruns is lucky Kathie refused — she might be one of the few women to be intimate with Robert Durst who lived to tell about it.

Durst was arrested a day before the March 14 final installment of an HBO documentary series about him, “The Jinx,” where he was caught on a microphone whispering to himself, “What the hell did I do? Killed them all, of course.”

Durst, 71, was charged with the murder of his confidante Susan Berman, who was killed execution-style at her home in Beverly Hills in December 2000.

The crime came just days before law enforcement was to grill her about Kathie’s disappearance on Jan 31, 1982.

Back then, Kathie Durst, 29, was living in mortal fear of her husband of 10 years.

Three weeks earlier, she was treated at a Bronx hospital for facial bruises. The couple had fought bitterly, Kathie told Schwank, and her husband pummeled her. Kathie was demanding a divorce and a $250,000 settlement — a modest amount for a real-estate scion with millions.

She was also fed up over Durst’s three-year affair with Prudence, the then-34-year-old sister of a Hollywood star and daughter of Maureen O’Sullivan — Jane in the Tarzan movies.

The slim brunette had also had her own brush with fame when she was 19 — accidentally ending up at the same Indian ashram as The Beatles.

John Lennon wrote “Dear Prudence” for her after he was dispatched to coax her out of her meditation hut. The lyrics — “Won’t you come out to play?” — were a comment on her fanatical devotion to the discipline.

“She’d been locked in for three weeks and wouldn’t come out, trying to reach God quicker than anybody else,” Lennon said. As Prudence herself noted years later, “I wanted to be immersed fully in the divine.”

Shortly after the song was released, Prudence married Albert Bruns, a New York teacher. By 1980, however, it was widely known that she was dating Robert Durst, the handsome heir apparent to a real-estate dynasty that owned most of Times Square.

“He was thrilled to be dating the woman who had been the inspiration for his idol John Lennon’s song,” according to author Marion Collins in “Without a Trace.”

After years of wandering between California and Texas, often dressed as a woman, Durst in 2001 killed and dismembered Morris Black in Galveston.

Two years later he was acquitted of murder, convincing a jury that he shot his elderly neighbor in self-defense.

At the end of his trial, Prudence told police investigators that she was worried Durst might come after her.

She said Durst became enraged when she tried to break off their relationship back in 1982, days before Kathie went missing, according to published reports.

John Lennon wrote “Dear Prudence” for her after he was dispatched to coax her out of her meditation hut.

Today, Prudence, 67, lives in the Florida Panhandle with her husband of nearly 50 years, off a dirt road in a mobile home hidden behind thick vegetation.

The home doesn’t even have a mailbox — seemingly the perfect place for a person who does not want to be found.

The mother of three and grandmother of four earned a Ph.D. in Sanskrit studies from UC-Berkeley and worked as a schoolteacher and film producer.

When visited by The Post last week, Prudence politely explained that she was teaching a class at home in transcendental meditation and that her student was waiting.

She then said, “I’m really sorry, but I have no comment.”

Additional reporting by Jay Granberg.