Nishimachi International School was established in 1949 by the late Tané Matsukata on the family property in the Azabu area of Tokyo. She had recently returned to Japan after seventeen years in the United States, where she received her education and spent the war years.

Miss Matsukata found Tokyo still badly scarred from the war while the slow process of rebuilding had begun. It was apparent to her that there was moral rebuilding to be done as well. In discussions with friends, who were mothers, she began to realize the important role that education would play in the reconstruction process. Together the ladies explored the alternatives and concluded that a new approach other than traditional education was needed, one that stressed the human side of learning and had peaceful coexistence with others as an objective.

Accordingly, Miss Matsukata and her friends decided to start their own school, along the lines of the progressive academy Miss Matsukata's mother had established for her and her siblings in the years before the war.

Learning a second language, in this case English, was step one in the educational process. The founders saw this as fundamental to extending the children's understanding beyond the boundaries of their own culture and into other cultures.

Tané Matsukata and her sister undertook the English portion of the curriculum. They hired a teacher for Japanese, and the school that was eventually to become Nishimachi opened its doors to its first four students in 1949.