Tanuma Okitomo was born in 1749 in Tokyo, Japan. His father was Tanuma Okitsugu (a chamberlain (sobashū) and a senior counselor (rōjū) to the shōgun Tokugawa Ieharu).
Tanuma served Shogun Ieshige Tokugawa. He was promoted to Wakatoshiyori (senior clerk) in 1783. In 1784 Okitomo was assassinated inside Edo Castle. He was killed in front of his father as both were returning to their norimono after a meeting of the Counselors of State had broken up. Okitomo was killed by Sano Masakoto, a hatamoto. The involvement of senior figures in the bakufu was suspected, but only the assassin himself was punished.