As his elder brother and lord of the domain Takatoshi KATAGIRI died with no heir to follow him in 1638, Tamemoto inherited the domain as matsugo yoshi (adopted as a son on the adoptor's deathbed), but the family's territory was reduced from 40,000 goku to 10,000 goku in exchange for the approval of his inheritance.
寛永15年（1638年）、先代藩主で兄の片桐孝利が嗣子無くして死去したため、その末期養子として後を継いだが、このとき所領を4万石から1万石に減らされてしまった。