The founder of the Mitsui zaibatsu, Mitsui Hachirobei Takatoshi, established shops for dry goods in Kyoto (the old capital) and Edo (now Tokyo) in 1673 to sell high quality kimonos. Ten years later he and his sons established money changing shops which soon were carrying out exchange operations for the Tokugawa Shogunate, the government of Japan. One operation Mitsui performed for the Shogunate was to convert taxes which were paid in rice into money available in Edo. Osaka was the principal commercial center of Japan at the time and Mitsui sold in Osaka the rice collected as taxes. Mitsui handled the transfer of funds from Osaka to Edo. Transporting money itself from Osaka to Edo was dangerous and expensive so Mitsui instead bought the promissory notes Edo merchants gave to Osaka and Kyoto merchants and transferred these to Edo for collection. Mitsui did not charge a fee for these important services to the Shogunate but instead was compensated by having the use of government funds for a period of months. For those months between the receipt of the rice tax and the payment of money in Edo Mitsui had, in effect, an interest-free loan which it could lend at interest to other borrowers. Eventually these financial services businesses became more important than the dry goods shops which were the original business of the Mitsui family.

Although providing services for the Shogunate was profitable for Mitsui there were risks. When the Shogunate got into political difficulties and needed funds it squeezed the political merchants like Mitsui who had been making profits from its patronage. After demanding substantial comtributions in the 1864-65 period from Mitsui and other political merchants, in 1866 it demanded a payment from Mitsui alone which was more 50 percent larger than Mitsui's operating assets.

Mitsui had to petition for a reduction in the amount the Shogunate was demanding. Mitsui founded a talented negotiator, Minokawa Rihachi, who had once been a servant of the Minister of Finance. Minokawa was uneducated and illiterate but he was a very sharp political operator. Minokawa got the payment demanded of Mitsui reduced by two-thirds. Furthermore the payment was split into three installments. Before the last two installments were due the Shogunate was overthrown.

However before its overthrow the Shogunate had asked that Mitsui set up in Yokohama a lending organization that would be funded by custom duties. Minokawa as a reward for his successful negotiation was selected to manage the Yokohama lending institution for Mitsui. Minokawa changed his name to Minomura for this new career.

When the Tokogawa Shogunate was overthrown Mitsui was selected by the new Meiji government to provide financial services and handle the creation of a new currency. Mitsui was also instructed to begin preparation for the creation of a central bank for Japan.

After Mitsui was well along in its plans to create a central bank the Meiji government changed its mind and called for Mitsui to collaborate with another business, Ono, in the formation of the central bank. Mitsui, Ono and Shamada were the three businesses which held government deposits and benefited from having the use of these funds interest free for a period of time. In October 1874 the government demanded that each of these three firms make a payment to the government equal to one-third of the amount that the government held with them. This payment was required within a couple of months which was too short a time for Ono and Shamada to collect on the loans they had out of government funds and they went bankrupt. Mitsui was able to survive because government funds were a smaller proportion of their holdings than they were for Ono and Shamada. Mitsui had also been more cautious in its lending than Ono and Shamada. But the key to Mitsui surviving is that its political operator, Minomura, had gotten advanced warning of the change in government policy and had longer to raise the payment required.

In 1876 Mitsui formed the Mitsui Bank, a major financial institution in Japan in the years ahead.