A few days after it was announced that the global commodities trader Glencore was to mount the biggest stock exchange float in British history, every major news organisation in the UK received a terse letter from a London law firm.

The letter was from Schillings, which is no ordinary corporate law firm: its lawyers describe themselves as reputation management experts, people who help clients "manage what is published and broadcast about them". They are particularly proud of their own reputation as the country's leading superinjunction specialists.

Glencore executives, the letter said, "are extremely private individuals", who expected scrutiny of their business activities, but not their personal lives. A warning followed about the "security risk" that could be posed by any reports about their homes or private lives.

Not all Glencore's board members are extremely private. A few days after Schillings' letter was dispatched, the group's new chairman, Simon Murray, gave one of his frequent media interviews, in which he offered his personal views on a number of matters, including asylum seekers – "people who claim to be running away from some place in Africa because they're being beaten up or something" – and his reluctance to employ young women, "because I know they're going to get pregnant and they're going to go off for nine months".

The executive whom Schillings had in mind when writing its letter was Glencore's chief executive, Ivan Glasenberg, a man so secretive that the Financial Times has described him as "one of the great enigmas of the corporate world".

In commodities, Glasenberg's name enjoys instant recognition. This is unsurprising, given his company's role in supplying the basic materials that heat, feed, move and house the world. Oil, gas, coal, aluminium, bauxite, nickel, iron ore, zinc, copper, grain, rice, sugar: Glencore and its subsidiaries have a hand not just in buying and selling all of these, but in producing, extracting and transporting them.

Glencore's market share is so large that it recorded revenues of $145bn (£89bn) last year and the flotation value is £37bn. Glasenberg, owning around 16%, will instantly become one of the world's richest men.

The shares started so-called "conditional dealings" on Thursday – proper trading does not start until next week – and had a far from sparkling start, moving only sideways even though they were priced keenly in an effort to get a strong start.

Because the company will leap straight into the FTSE 100 index, those investments that track the UK's biggest quoted companies will be obliged to buy its stock, with the result that the company's fortunes will have a bearing on the pension funds of millions of people.

But so jealously has Glasenberg guarded his privacy that his name means nothing to the man on the street. For years he has avoided speeches and, until recently, had given only one interview – to his old university magazine. If you live outside the world of commodities trading or corporate finance, Ivan Glasenberg is probably the Most Important Businessman You Have Never Heard Of.

He was born in January 1957 in South Africa, one of four children of Samuel Glasenberg, a luggage manufacturer and importer born in Lithuania, and his wife, Blanche, a South African.

The family home was in Illovo, a comfortable, tree-lined northern suburb of Johannesburg. Glasenberg attended a state school in nearby Hyde Park, where one teacher recalls him as an independent thinker "who did not always accept that the teacher was correct".

His parents were keen to see him forge a career in business, and he studied accounting at the University of the Witwatersrand before serving a form of apprenticeship with the Johannesburg auditing firm Nexia Levitt Kirson.

Glasenberg was athletic – even today he tries to run or swim every day – and excelled at a particularly unusual sport: race walking. By his early 20s he was the country's junior champion, and hoped to compete in the Olympics. Realising that, as a South African during apartheid, this could not happen, he considered applying for Israeli citizenship. Today, the fact that he could not compete in the Olympics is said still to rankle with him.

Like every other young white South African male, he had to do national service. Friends say he describes the year he spent as an army clerk, many miles from the frontline, as his "brain-dead year", when he did not once need to think for himself. Tony Leon, the South African ambassador to Argentina, who shared accommodation with him, said: "We weren't the world's most conscientious soldiers: getting out of guard duty was the order of the day. None of us took our duties particularly seriously. And if he had any political views, they weren't apparent."

Another person who knew Glasenberg at this time said: "I don't know his views but I would guess he was anti-apartheid. He left South Africa as soon as he could, which is congruent with many Jewish South Africans of that time."

Glasenberg travelled to Los Angeles to study for a master's in business administration at the University of Southern California's business school. Years later he told its magazine that it had been "an enormous cultural shock" to leave South Africa. "I stopped focusing on people being different and I started treating everyone the same way."

In the same interview he explained that he had become intrigued by commodities trading at Witwatersrand when he learned about the global trade in one raw material: wax. "I observed a man sourcing candle wax from South America and selling it to Japan. I thought: 'That's unbelievable. Talking on the phone in his office, that man made money moving candle wax from one country to another' It really interested me."

It was the only interview Glasenberg gave before Glencore announced its float. Even that appeared one too many. When it appeared on its website, Glasenberg asked his alma mater to take it down. Today, members of faculty refuse to talk about him, saying they know he values his privacy, so it would not be in the university's interest to do so.

On graduating in 1983, Glasenberg applied, successfully, for a job in New York, working for the man who was at that time the biggest commodities trader in the world: Marc Rich.

Glasenberg never did get to work in New York. Just as he was about to join the company, Rich realised that he was going to have his collar felt by the US federal authorities, and fled to Switzerland, never to return. He was subsequently charged with racketeering, evading millions of dollars in taxes and trading with the enemy: the Ayatollahs' Iran.

Rich was America's most-wanted white-collar criminal, and his picture adorned the FBI's list of top-10 fugitives alongside that of Osama bin Laden. He stayed on the list until Bill Clinton's controversial decision to pardon him during the final hours of his presidency in 2001. However, when Rich went on the run, Glasenberg was told there was still a job for him at Marc Rich & Co: back in Johannesburg.

At that time, South Africa was at the heart of what Rich would later tell his biographer, Daniel Ammann, was the "most important and most profitable" part of his business. He made an estimated $2bn supplying oil to the apartheid regime. Glasenberg, who began work as a junior member of staff in the coal division, was aware of the oil trading but believed there was nothing wrong with it and had no idea whether or not any embargo was being broken. In the event, by the time the UN adopted an international oil embargo on South Africa at the end of 1987, Glasenberg had moved on, working for Rich first in Sydney and then Beijing, selling coal across the far east.

In 1991 he was brought to head office in Switzerland as head of the coal division. He had caught Rich's attention. Rich told the Guardian he believes Glasenberg to be a brilliant commodities trader. "I liked him right away. He is an excellent analyst, very intelligent and hard-working. Without a doubt he is the strong man at Glencore."

Over the next couple of years, Glasenberg became a trusted member of the inner circle that was known, perhaps inevitably, as the Rich Boys. They had a reputation for being pragmatic, aggressive in their trading and deal-making, and very secretive. They were by no means reclusive – a large network of influential contacts was essential – but they were said to have no wish to draw attention to the deals they struck.

"They have profited from being extremely secretive'" says Ammann. "The sort of people they do business with do not want their deals in the spotlight."

As well as trading with South Africa and Iran, Rich was dealing with Castro's Cuba and giving Mossad an occasional helping hand. At the end of 1993 he lost control of the company when a disastrous attempt to corner the world zinc market led to a number of the Rich Boys insisting he give up his majority stake. After a management buyout, Marc Rich & Co was renamed Glencore. Glasenberg was appointed chief executive in 2002, and until now the company has been run as a private partnership.

Today, he and his colleagues are eager to play down the Rich connection. Glencore's website says the company was founded in 1974, but there is no mention of the founder's name.

While Glasenberg is already a wealthy man, his lifestyle is by no means opulent. He is said to have just one home, a discreet modern villa in a pretty village near Zurich, not far from the Lindt chocolate factory. He appears to be motivated more by a determination to succeed – to be the best commodities trader, running the best business – than to be even more wealthy. Indeed, one person who knows him well say he is driven by anxiety that he may not succeed; that fear of failure is "the biggest fear of every minute" of his life.

Taking Glencore to the next level, however, giving it the chance of even greater success through a series of acquisitions, requires the $11bn of funds that will be raised through the flotation. And that will come at a high cost: his privacy. Glasenberg can expect far greater public scrutiny, and so can Glencore.

This will not be easy. Secrecy, says Ammann, "is in Glencore's DNA". It is also highly valued by its chief executive. Glasenberg prizes the anonymity that he and his wife and two children enjoy – and the fact that until now many of his old school friends in Johannesburg had no idea how much he earns.

Despite Schillings' letter, he knows this is about to change. Glasenberg, says one person who knows how much he anguished over the decision to take Glencore public, is well aware that he "has crossed the Rubicon".