Son was an early investor in internet firms, buying a share of Yahoo! in 1995 and investing $20 Million in the Ali Baba stake in 1999. Although SoftBank's stake in Yahoo! had dwindled to 7%, Son established Yahoo! BroadBand in September 2001 with Yahoo! Japan in which he still owned a controlling interest. After a severe devaluation of SoftBank's equity, Son was forced to focus his attention on Yahoo! BB and BB Phone. So far, SoftBank has accumulated about $1.3 billion in debt. Yet, Yahoo! BB acquired Japan Telecom, the then third largest broadband and landline provider with 600,000 residential and 170,000 commercial subscribers. Yahoo! BB is now Japan's leading broadband provider.
On March 17, 2006, Vodafone Group announced it had agreed to sell Vodafone K.K. to SoftBank for approximately 1.75 trillion Japanese yen (approximately US$15.1 billion). On April 14, 2006, SoftBank and Vodafone K. K. jointly announced, that the brand and company name Vodafone will be changed to a "new, easy-to-understand and familiar company name and brand". Masayoshi Son is the CEO (Representative Director) of Vodafone K. K.
In 2011 Masayoshi Son pledged to donate 10 billion yen ($120 million) and his remaining salary until retirement to help support victims of 2011 Tōhoku earthquake and tsunami.
Son was named the world's 45th most powerful person by Forbes Magazine's List of The World's Most Powerful People in 2013.
In July 2016 SoftBank announced its plan to acquire ARM Holdings for £23.4 billion ($31.4 billion) which had to be the biggest ever purchase of a European Technology company. In September 2016 SoftBank announced that the transaction is complete. The total acquisition price was approximately £24 billion ($31 billion).