Here are five key facts about his life and career:

Works as accountant

Born in Romania in 1931 to a poor Jewish family, Mandel developed a strong interest in mathematics but lacked the financial means to pursue an academic path.

Instead, he worked as an accountant to support his family in the late 1960s. Earning just 360 lei (about US$10) a month, he found it difficult to support his family and, as he told NPR's Planet Money in a podcast aired in 2016, needed a way to "get some serious money, quickly."

Many Romanians in similar circumstances turned to crime, but Mandel, who described himself as a "philosopher-mathematician," saw another option: the lottery.

Wins first jackpot in Romania

Spending much of his spare time studying probability theory and the writings of Italian mathematician Leonardo Fibonacci, Mandel eventually developed a "number-picking algorithm" based on a method he called "combinatorial condensation," according to The Hustle.

The idea was simple. He first selected a group of 15 numbers that he believed had a strong chance of containing the six winning numbers. Instead of buying all 5,005 possible six-number combinations from those 15 numbers, his formula reduced the total to 569 carefully selected tickets.

Romanian-born mathematician Stefan Mandel. Photo from X

In March 1965, Mandel persuaded a friend to buy lottery tickets with him and split any winnings 20-80 because he did not have enough money to fund the plan himself.

Their first attempt was successful. The pair won the jackpot and several smaller prizes worth about $20,000. Mandel received roughly $4,000, equivalent to four years of income for an average Romanian family at the time.

"I’m a weekend mathematician, an accountant without too much education," he later told a Romanian magazine. "But mathematics properly applied can guarantee a fortune."

NPR's Planet Money producer Alex Goldmark, who interviewed Mandel, said his success was built on years of research rather than luck.

"He doesn't just go out and buy random tickets," Goldmark told Business Insider. "He's a pretty natural math whiz. He goes to the library and he starts reading math paper after math paper after math paper, and he comes up with a formula for buying blocks of tickets that he thinks should guarantee him a prize."

Develops formula and earns millions

Mandel used the winnings to move his family to Australia, where he attracted more investors and refined his lottery strategy. His system ultimately relied on six basic steps, according to The Independent:

1. Calculate the total number of possible combinations.

2. Find lotteries where the jackpot is at least three times greater than the cost of buying every combination.

3. Raise enough money to purchase all combinations.

4. Print millions of tickets covering every possible outcome.

5. Deliver the tickets to authorized lottery dealers.

6. Collect the winnings and repay investors.

Throughout the 1980s, his syndicate waited until a jackpot exceeded the cost of covering every possible combination before buying massive numbers of tickets. According to the draft, the group won 12 lotteries and about 400,000 smaller prizes across Australia, including a $1.1 million jackpot in 1986.

"Everyone said to me: You can not, you will not succeed! " Mandel told a Romanian newspaper at the time. "Now, the voices that have long [cast me as] a dreamer have been silenced."

His automated system eventually attracted the attention of authorities. Although investigators found that his methods were legal, lottery operators later introduced rules banning bulk ticket purchases and computer-generated tickets.

Goes bankrupt

As regulations changed, Mandel found it increasingly difficult to apply his strategy in Australia and turned his attention to the Virginia Lottery in the U.S.

The game used numbers between one and 44, reducing the total number of possible combinations compared with many other lotteries. Mandel established Pacific Financial Resources and its International Lotto Fund, raising millions of dollars from investors to finance the operation, the New York Post reported.

Working from a warehouse in Melbourne, his team of 16 employees spent three months printing seven million lottery tickets, which were shipped to an associate in the U.S. The operation succeeded, producing another jackpot victory along with numerous smaller prizes.

Although his methods remained legal, they triggered a four-year investigation. Mandel was eventually cleared of wrongdoing, but the lengthy legal battle left him in financial difficulty. He earned millions from the lottery, but some investors received less than they had expected. In 1995, Mandel filed for bankruptcy.

"I’m a man who takes risks, but in a calculated way," Mandel told Romanian newspaper Bursa in 2012. "Trimming my beard is a lottery: There is always the possibility that I’ll cut myself, get an infection in my blood and die — but I do it anyway. The chances are in my favor."

Faces fraud conviction

In the early 2000s, Mandel returned to Israel in an effort to replicate his business success but came under investigation by the Israel Securities Authority.

After being indicted, he left his wife and children in Israel and moved to London in 2002, as reported by Israel’s newspaper Ynet Global.

In 2004, Mandel and his partner were convicted on fraud charges after authorities alleged they failed to provide investors with the actual odds of success they had promised. He was sentenced to 10 months in prison and fined NIS100,000 (US$29,000).

For years, Mandel sought to overturn the conviction. In 2006, his appeal was rejected because he was considered a fugitive. In 2018, Mandel submitted another request for a retrial, arguing that he had new evidence showing the investigation had been driven by improper motives and conflicting interests. He also alleged violations of due process.

In May 2023, Mandel returned to Israel after more than two decades abroad and sought a retrial over his 2004 securities fraud conviction. Later that year, Israel's Supreme Court rejected the request, leaving the conviction and his 10-month prison sentence in place.