Kanamori was born in 1886 in Aichi Prefecture. He graduated from Tokyo University in 1912 with a degree in Law, and started work for the Ministry of Finance. In 1924, he was appointed as Director of the new Cabinet Legislation Bureau (the first person to hold this post) and he subsequently became Director-General of the Bureau a decade later. He was, however, pressured into resigning only two years later, due to sympathies with the controversial "Emperor Organ" theory proposed by Tatsukichi Minobe. Nevertheless, he remained a respected constitutional scholar, who had published several books on the Meiji Constitution. He was consulted on the wording of the new constitution by Toshio Irie
His election to the House of Peers in 1946 marked the start of his return to politics. He was elected as Minister of State in the first cabinet of Shigeru Yoshida, and in this capacity argued forcefully in favour of the new postwar constitution. In the course of 114 days of debate, Kanamori responded to over a thousand questions, with extensive responses that took up to one and a half hours. After the implementation of the new legislation, Kanamori helped to form the Kenp? Fuky? Kai (Constitution Popularization Society) in December 1946, and published books and tracts to spread awareness of the new laws throughout the population.