Truth about Warren Buffett's 'unconventional' marriage: The open secret about his 'closest advisor' and his wife's stunning letter to another woman revealed

For years, many of Warren Buffett’s friends received a Christmas card from the Oracle of Omaha signed by three people: ‘Warren, Susie, and Astrid.’

The signatures belonged to Buffett, his wife Susan, and Astrid Menks, who moved into their home while the Buffetts were still married.

On Friday, Buffett, 96, stepped down as chairman of Berkshire Hathaway, passing the top job to his son Howard. ‘Father Time always wins,’ he told shareholders. ‘He has, however, been generous with me.’

Over 61 years, Buffett transformed the struggling textile maker into a $1 trillion investment giant by following a set of staid principles that projected simplicity, patience and self-control.

His private life was a bit more complicated.

Buffett married Susan Thompson Buffett in 1952. Daughter Susan Alice was born in 1953; Howard and Peter followed in ’54 and ’58. Then things took a turn in 1973, when Buffett first started buying shares in The Washington Post Company.

Buffett, then 46, grew close to its steel-spined publisher, Katharine Graham, a widow 13 years his senior.

Alice Schroeder’s 2008 biography, The Snowball: Warren Buffett and the Business of Life, describes the relationship as an affair.

Buffett personally chose Schroeder to write his life story. He gave her thousands of hours of interviews and access to his files, family and associates. He even told her to use the less flattering version when accounts conflicted. A spokeswoman for the publisher later said Buffett reviewed the book but changed nothing. After the book was published, however, Schroeder said their relationship cooled.

According to Schroeder, Buffett kept clothes at Graham’s Washington home and spent time with her at Martha’s Vineyard. She tried to improve his wardrobe by taking him to meet the fashion designer Halston. They didn’t appear to exactly keep the nature of their relationship a secret, either. At charity events, Graham would reportedly toss him her house key.

Carol Felsenthal, journalist and author of the unauthorized Graham biography Power, Privilege and the Post, separately heard from members of Graham’s Washington circle who believed the relationship was sexual.

‘He became her closest advisor, and she completely relied on him,’ Felsenthal tells the Daily Mail. ‘Whatever Warren said was gospel.’

Felsenthal says at least two people she interviewed believed Buffett and Graham were lovers. But she draws a firm line between what her sources believed and what she can prove.

‘Whether she and Warren Buffett cuddled and had sex, I have no idea,’ she says.

Whatever happened physically, the closeness had consequences at home. Schroeder writes that Susan Buffett was furious and humiliated by the attention they attracted, later sending Graham a letter effectively granting her permission to pursue a relationship with her husband. Graham, according to Schroeder, showed the letter to friends as though it cleared her.

Susan was also pursuing a life outside the marriage. Schroeder reports that she had begun a romance with her tennis coach, John McCabe. In 1977, Susan moved into a small apartment in San Francisco, with McCabe taking another apartment nearby.

But she and Buffett did not divorce. Instead, their marriage stretched into something difficult to categorize. Susan wanted a life of her own but remained deeply tied to her husband. Warren was devastated by her departure and, according to Schroeder, called her every day, sometimes weeping.

He also had difficulty taking care of himself, and Susan knew it. Before settling in San Francisco, she asked her friend Astrid Menks, who worked at the French Café in Omaha where Susan sang, to check on him.

‘I called Astrid,’ Susan later recalled. ‘I said, Astrid, will you take Warren, make him some soup, go over there and look after him?’

Astrid did. By 1978, she had moved into Buffett’s Omaha home.

Susan remained Warren’s wife and an important emotional presence in his life. Astrid became his everyday companion. The two women developed their own relationship, and Buffett’s daughter Susie later said they ‘really loved each other.’

‘In contemporary terms, we might call this polyamory despite the individuals never having used that label themselves,’ Mimi Schippers, a professor of sociology at Tulane University who studies polyamory and non-monogamy, tells the Daily Mail.

Terri Conley, a professor of psychology and women’s and gender studies at the University of Michigan who studies departures from monogamy, goes further.

‘This sounds most like polyamory,’ she says. ‘That is, it wasn’t just about sex. The third partner was integrated into their lives in multiple ways.’

That does not mean the three were a ‘throuple,’ or that Susan and Astrid had a sexual relationship. There is no evidence of that.

By 1987, Schroeder writes, the intensity of Buffett and Graham’s relationship had cooled, though they remained close friends until Graham’s death in 2001.

Warren and Susan never divorced. During Susan’s cancer treatment, he traveled regularly to San Francisco to be with her. She died in 2004 at 72.

Two years later, Buffett married Astrid in a 15-minute civil ceremony at his daughter’s Omaha home. Dinner afterward was at the casual dining franchise restaurant Bonefish Grill.

Their daughter Susie had a succinct verdict on the family arrangement. ‘Unconventional is not a bad thing,’ she said. ‘More people should have unconventional marriages.’

Buffett himself was less celebratory about how it had begun. Looking back on Susan’s departure, he told Schroeder that it’d been preventable and called it the biggest mistake he ever made.

‘It was definitely ninety-five percent my fault,’ Buffett said. ‘I just wasn’t attuned enough to her, and she’d always been perfectly attuned to me.’

Then, more simply: ‘It shouldn’t have happened.’