After becoming Mitsubishi's fourth president in 1916, Koyata Iwasaki initiated a wave of organizational reforms, spinning off various business divisions as joint-stock companies under the umbrella of Mitsubishi Goshi Kaisha. By making stock available to the public and bringing in outside capital, Koyata not only secured funds for business expansion, but he also sought to contribute to society by turning the companies into pubic assets. The old Mitsubishi Shoji and many other "Mitsubishi" companies were launched as separate companies in this way.

Meanwhile, Koyata concluded it would be best to reintegrate Mitsubishi's shipbuilding and aircraft manufacturing arms in light of the many similarities they shared in terms of technology and facilities. There was considerable opposition to this merger, both from inside and outside the company, but Koyata did not back down: "There is an urgent need to integrate these industries, especially when you consider their importance to the nation." Mitsubishi Heavy Industries was subsequently launched in 1934, bringing together operations involving shipbuilding and the manufacturing of heavy machinery, aircraft and other industrial equipment.

In 1941, the Pacific War broke out. Many Mitsubishi factories sustained damage as the war intensified. Koyata, however, continued to visit factories because he believed that encouraging frontline workers was one of his primary responsibilities as president.

After Japan was defeated in 1945, the General Headquarters of the Supreme Commander for the Allied Powers ("GHQ") called for the "voluntary" dissolution of Japan's large, family-controlled conglomerates. The other conglomerates complied immediately, but Mitsubishi held out. Koyata remarked, "We have done nothing to be ashamed of." Although his health had deteriorated under the strain of the wartime years, Koyata continued to insist that Mitsubishi's voluntary dissolution was out of the question. However, Koyata was eventually hospitalized for his illness and Mitsubishi subsequently acknowledged that its dissolution was unavoidable.

Before his death, Koyata left the following note: "No matter what happens to the Mitsubishi organization, we will forever remain united and I believe all the various Mitsubishi enterprises will continue to be operated in accordance with the Mitsubishi spirit." Koyata tenaciously stood by his convictions and his sense of duty as Mitsubishi's leader never wavered, even in the face of great change. Koyata was the only president of Mitsubishi to have been born in Tokyo, but throughout his life, he demonstrated the kind of grit and fortitude that had typified his predecessors from Tosa (modern-day Kochi).