Takanaga Mitsui was born in Tokyo, Japan on February 28th, 1892. He attended Camp Awosting for several years, beginning in 1909. Takanaga Mitsui was a member of the powerful Mitsui family. His clan founded the House of Mitsui, the largest merchant house of the Tokugawa period (1603-1867). In the early 20th century, Takanaga’s family founded the Mitsui group, a massive business combine. Takanaga Mitsui attended the University of Michigan and received a Bachelor of Sciences (B.S.) from Dartmouth College in 1915. At Dartmouth, Takanaga contracted acute appendicitis. College President Ernest Fox Nichols and his wife helped him receive medical treatment. The couple hosted a delegation of top Japanese diplomats, who came to make sure the heir to the Mitsui Group was in good hands. Takanaga's family sent President Nichols a temple bell as a token of their gratitude.
Takanaga Mitsui returned to Japan and was employed by a family-owned business, Mitsui Mining Company Ltd. Takanaga's company produced domestic coal, coke, and cement, and was, for a time, the largest coal mining company in Japan. During World War II, several Mitsui group companies used Allied prisoners of war for slave labor. The prisoners were subjected to brutal treatment by Mitsui Group employees. One of the former prisoners, U.S citizen Lester Tenney, sued the Mitsui Group in 1999 for punitive damages and compensation. A Federal judge dismissed the case, citing the 1951 peace treaty between the U.S. and Japan which barred private claims against Japan. In 2010, the Japanese government issued a formal apology to Tenney. The Mitsui Group remained silent.
Takanaga Mitsui married Hiroko Shimazu (a member of his sprawling clan) and had two sons (Mamoru and Takanobu) who both attended Dartmouth, graduating in 1943 and 1958.
At the time of his June 25, 1917 WWI draft card, Taganaka Mitsui was living at 150 W.79th Strret in New York City. He stated his occupation was importer/exporter. He also stated that he had been in Havana Cuba between June 5th, 1917 and June 10th, 1918.