[4] As an entrepreneur, he achieved notability in PC software distribution, computing-related book and magazine publishing, and telecommunications in Japan, starting in the 1980s and booming throughout the 1990s and 2000s.

[18] He had for many years the distinction of being the person who had lost the most money in history (more than $59bn[19] during the dot-com crash of 2000 alone, when his SoftBank shares plummeted),[20] a feat surpassed by Elon Musk[21][22][23] in the following decades.

His father and other Koreans illegally built their houses on land that was owned by Japan National Railways, which caused them trouble with the authorities.

[29] His father raised pigs and chickens on that land, and started an illegal sake business that eventually became successful enough for his family to become the first people in town to own a car.

[32][33] At age 19, Son became confident that computer technology would ignite the next commercial revolution after being impressed by a microchip featured in a magazine.

With the help of some professors including Forrest Mozer, Son created an electronic translator that he sold to Sharp Corporation for $1.7 million.

[47][42] In 2020, SoftBank Group agreed to sell U.K. chip designer Arm Limited to U.S. chip-maker Nvidia in a cash and stock deal initially worth $40 billion.

Announcing the deal, SoftBank said the combination of Arm and Nvidia would create a computing company "that will lead the era" of artificial intelligence.

[57] In March 2018, it was announced that Son was investing in the biggest ever solar project, a 200GW development planned for Saudi Arabia as part of its Vision 2030.

[65][66] As of 2020, the first fund had invested in 88 companies including Coupang,[68] Didi,[69] Doordash,[70] Fanatics,[71] Grab,[72] Oyo,[73] Paytm[74] Uber,[75] and WeWork,[76] but had experienced an awkward fall from grace[77][78][79][80] as the COVID-19 pandemic and a Chinese regulatory crackdown[81][82] accelerated the exposure of the Japanese investment management conglomerate's portfolio weaknesses.

[85] In addition, he raised his global profile as stock investor since starting Softbank Vision Fund in 2017, creating an unprecedented investment vehicle of almost $100 billion to back technology startups.

But by 2021, he was still struggling to persuade investors of the value of his efforts, in part because of major losses with companies such as WeWork, OneWeb, Wirecard,[86] OYO Rooms, Katerra[87] or Greensill Capital, and SoftBank Group's own stock chronically traded far below the value of its assets reflecting a discount[88] associated to tax liabilities, risk, past performance, losses, performance fees and high probability of occurrence of several haircuts given Son's poor track record while running the Vision Fund[89] and high enthusiasm for investing vast sums in loss-making companies at eye-popping valuations.

[95][96][97] In 2022, SoftBank Vision Fund posted a record 3.5 trillion yen loss ($27.4 billion) for its financial year ended on 31 March 2022 as the valuation of its stock portfolio plummeted.

[101] In August 2022, Masayoshi Son said he was "embarrassed" and "ashamed" when asked to talk about the way he had run the SoftBank Vision Fund[102][103] and Barron's characterized the fund as a "failed experiment"[104] while The Wall Street Journal called SoftBank a "big loser"[41] and Bloomberg elaborated on "Masayoshi Son's broken business model".

[105] By November 2022, according to the Financial Times, Masayoshi Son personally owed SoftBank $4.7bn because of growing losses on the Japanese conglomerate's technology bets, which have also rendered the value of his stake in the group's second Vision Fund worthless.

This debt on side deals he set up at SoftBank Group Corp. to boost his compensation, as losses mounted at its core Vision Fund venture capital arm, sparked controversy due to corporate governance concerns, but Son insisted that there wasn't any conflict of interest.

[109] Son was named chairman of The Stargate Project, an American artificial intelligence infrastructure joint venture formed by SoftBank, OpenAI, Oracle Corporation and MGX, in January 2025.

[110] Son met his wife, Masami Ohno, the daughter of a prominent Japanese doctor, while both were students at the University of California, Berkeley.

His youngest brother, Taizo Son, is a serial entrepreneur and investor, having founded GungHo Online Entertainment and the venture capital firm Mistletoe.

[114] When he went to the United States at 16 to attend high school and then the University of California Berkeley, he decided to use his real Korean surname.