Mori was born in the Nagata-chô neighborhood of Kagoshima in 1847. In 1865, at the age of 19, Mori was one of a number of students sent by Satsuma han secretly to England for study. While in England, he studied naval surveying. Mori then traveled to the United States in 1867/7, and returned to Japan the following year, in 1868/6. In 1869, Mori called for the prohibition of general private ownership of swords, and resigned his official post over the associated controversy. He then returned to Kagoshima, and opened an English-language school.
He was sent to Washington DC in 1870 as Japan's first minister to the US, and while there oversaw a number of surveys of the American educational system. He later served terms as ambassador to China, vice-minister of foreign affairs, and ambassador to England, among a number of other positions, before becoming Minister of Education in 1885. He held that position until his assassination, at his official residence, on the morning of the promulgation of the Meiji Constitution, February 11, 1889.
As Minister of Education, Mori shaped the Japanese education system based largely on the Prussian model, and with the ideal in mind that public education "should produce men to serve the nation." He was also a supporter of women's rights, monogamy, and freedom of belief; his assassination is said to have been motivated by a belief that Mori had been irreverent or disrespectful during a visit to Ise Shrine.