Regarding the origin of the Tosa Yamauchi clan into which Katsutoyo was born, the content submitted by the Tosa Domain for the "Kansei Choshu Shokafu" (a record of family trees of samurai warriors of the Edo bakufu) produced in the latter half of the Edo period stated that they were the descendants of the Sudo Yamanouchi clan, who were the descendants of FUJIWARA no Hidesato.
一豊の生まれた土佐山内氏の出自については、江戸時代後半に作成された『寛政重修諸家譜』に土佐藩が提出した内容によれば藤原秀郷の子孫である首藤山内氏の末裔である。