Originally from the province of Shinano, he went to Uesugi Kenshin for sanctuary along with Murakami Yoshikiyo after being attacked by Takeda Shingen. He became a vassal of Kenshin and fought in the Fourth Battle of Kawanakajima in 1561. But he was a better diplomat than commander, and Kenshin used him as such.
He supported Uesugi Kagekatsu in the Otate no Ran after Kenshin's death. He succeeded Kawada Nagachika as supreme commander of Ecchuu after the latter's death and became the master of Uozu Castle. When Uozu Castle fell to Sassa Narimasa in 1583, he became master of Kaidu Castle. He also served as negotiator for Kagekatsu, but eventually fell out of Kagekatsu's favor. However, Toyotomi Hideyoshi valued Mitsuchika's skills highly, eventually taking him into the Toyotomi Clan and bestowing the Toyotomi name upon him.