Mitsubishi Bank was founded in 1880 by a former samurai, Iwasaki Yatarō, and was a core member of the Mitsubishi Group of companies. It merged with The Bank of Tokyo in 1996 to form The Bank of Tokyo-Mitsubishi, Ltd., which at that point was the world's largest bank in terms of total assets. In 1998, upon merger of the then second and third largest Swiss banks, Union Bank of Switzerland and Swiss Bank Corporation, the newly created UBS AG became the second largest in the world at the time, behind only the Bank of Tokyo-Mitsubishi. The Bank of Tokyo had historically focused on foreign exchange business since its foundation as the Yokohama Specie Bank in 1880, while Mitsubishi had had a stronger focus on domestic corporate and retail banking. Both banks were relatively healthy in the wake of the Japanese asset price bubble. Until the Tokyo-Mitsubishi merger took place in 1996, Sanwa Bank, which was based in Osaka and was the anchor of the Sanwa Group keiretsu, had been considered the strongest bank in Japan, and it had aimed to be the world's largest bank during the "bubble era". By 2000, however, Sanwa was the fourth largest bank in Japan. It entered into merger talks with two other large banks, Asahi Bank and Tokai Bank, to create the world's third-largest bank by assets. Asahi (now part of Resona Holdings) pulled out of these talks later that year. By 2001, The Tokyo Trust & Banking Co. had been added to the merger group and the combined company was to be called United Financial Holdings of Japan. The merger was completed in 2002 and the new bank was officially named UFJ Bank Ltd. UFJ was headquartered in Nagoya, the historical headquarters of Tokai Bank. During its short life, it was plagued by bad debt problems and by infighting between the employees of its predecessor companies. The holding companies of BTM and UFJ agreed to merge in 2005, forming Japan's largest bank by assets and market capitalization. This led to litigation between BTM and Sumitomo Trust & Banking Co., which had previously agreed to an alliance with UFJ under which it would take over UFJ's trust banking operations. BTM and UFJ settled their dispute for 2.5 billion yen in late 2006. The merger of the two bank holding companies was completed on October 1, 2005, creating the Mitsubishi UFJ Financial Group. The core banking units of MTFG and UFJ Holdings, the Bank of Tokyo-Mitsubishi, Ltd. and UFJ Bank Ltd., respectively, continued to operate separately until January 1, 2006, when the two units combined to form The Bank of Tokyo-Mitsubishi UFJ Ltd.

Mitsubishi Bank and the Bank of Tokyo each had significant banking subsidiaries in California (Bank of California and Union Bank respectively) before their merger. At the time of the merger, these U.S. banks also merged to form UnionBanCal Corporation. BTM listed UnionBanCal on the New York Stock Exchange in 1999. In 2008, BTMU purchased all of the outstanding shares of UnionBanCal. BTMU moved its New York-based banking operations to Union Bank and renamed the company MUFG Union Banking 2014.