There were no golden thrones, champagne flutes or ice sculptures at Nebraska's wedding of the year. The groom wore an everyday business suit, the ring was a discount purchase through his own jewellery company and the reception was a meal at the Bonefish Grill, an America-wide seafood chain.

Warren Buffett, the world's second richest man with a fortune of $44bn (£23bn), married his long-term mistress this week in a quiet ceremony at his white stucco house in the prairie city of Omaha, putting an end to an unorthodox relationship that has intrigued American society.

Buffett's Latvian-born bride, Astrid Menks, is 60 to her husband's 76. The pair have been living together since the late 1970s. Nothing unusual about that, except that Buffett had an apparently close and loving marriage to another woman, Susan, until her death in 2004. The trio even sent out Christmas cards signed "Warren, Susie and Astrid".

Andy Kilpatrick, a biographer of Buffett, says the billionaire is a man who yearns female companionship: "He's a fellow who's brilliant but he needs people around him who can take care of some needs. He doesn't really do household chores, he's got no mechanical brain. He can barely even turn on a light switch."

Known as the Sage of Omaha for his legendary stockpicking prowess, Buffett is in many ways a model of sobriety - he drinks cherry Coke rather than alcohol, has a number plate reading "thrifty" and entertains friends such as Bill Gates and Bill Clinton with endless games of bridge.

A father of three, he first encountered Menks in the mid 1970s when his first wife resumed a long dormant career as a nightclub singer by taking a job at the French Café, an upscale Omaha nightspot. Menks was working there as a waitress, delivering a faux-French menu with dishes such as "le surf and turf".

After a quarter of a century of marriage, Susan opted in 1977 to move to San Francisco to pursue her career - partly, friends say, because she felt smothered by the attention her increasingly famous husband was receiving.

Their separation was only partial. The couple continued to see each other regularly, socialising and working together.

Susan asked several friends in Omaha to look after her husband - including Menks, who began discreetly delivering him home-made soup and moved in with him a year later.

The two women proceeded to "share" the billionaire. As his wealth grew, Susan Buffett took on the task of managing her husband's charitable foundation. Buffett spent a good deal of time with his wife - they often appeared at functions together.

Menks would pack him off to see her, apparently without a trace of jealousy, and would sometimes appear beside her smiling, talking and even holding hands.

In his book The Making of an American Capitalist, the author Roger Lowenstein wrote: "From the day she moved in, Astrid knew that Buffett did not envision remarrying, and that he was still attached to his wife. She made a home for him, yet she would see him off with neatly laundered shirts when he left town to be with Susie."

The arrangement appeared to work well. Buffett's daughter, Susan Jr, told the New York Times: "Unconventional is not a bad thing. More people should have unconventional marriages."

It is a sign of the esteem with which Buffett is held in America that his relationships have generally been politely ignored by the media. Buffett's first wife roamed the world espousing causes including contraception, population control and advocacy for womens' right to have abortions.

Astrid, meanwhile, kept the home fires burning and developed a keen interest in antiques. A petite blonde woman with a sharp sense of style, she is often to be seen searching Nebraska's shops for hidden bargains.

When Susan became ill, Buffett spent every weekend with her in San Francisco. He was at her side when she died two years ago - by which time she was the world's 17th richest woman in her own right.

According to those who know him, Buffett is not an easy man to live with. One acquaintance said: "He's just all business. He lives, breathes and sleeps business, apart from a game of bridge in the evening."

Originally based on insurance, Buffett's Berkshire Hathaway firm now has a vast range of investments ranging from Fruit of the Loom underwear to Coca-Cola, Gillette and the utility GE Electric. One explanation for his reluctance to sever ties with his first wife is practicality - a divorce would have had potentially damaging implications for his empire.

Menks shows little sign of interest in his money. Buffett's daughter noted that she was with him "for all the right reasons", adding: "She loves him and takes care of him. If Warren didn't have a cent, she'd be with him."

She has taken on a handful of local causes including support for Omaha's Henry Doorly zoo, an institution strong in conservation which recently discovered 11 new species of lemurs.

Kilpatrick says: "People see her around town. She's very much a bargain hunter and a recycler. She'll be out looking for bargains for the house, shovelling snow or doing errands for him."

The couple still live in a home which Buffett bought for $31,500 in 1958. Their 15-minute wedding ceremony was overseen by a judge, Patricia Lamberty. The only witnesses were Buffett's daughter and Menks's sister, and there were no paparazzi: the world media rights to the nuptials were handed to the Omaha World-Herald, which was tipped off by the family after the event.

It was a typically low-key affair for the notoriously scruffily dressed Buffett, who avoids glamour at all costs. When he was once inducted into the Omaha Business Hall of Fame, he quipped that he wanted to thank his hair stylist, wardrobe consultant and personal trainer - but on seeing their handiwork, they all wanted to remain anonymous.