After Toshimasu gained favor with Kagekatsu UESUGI and his chief vassal, Kanetsugu NAOE, he served the Uesugi Clan from 1598, when Kagekatsu was transferred from the Province of Echigo to the 1.2 million koku Domain of Aizu, until the Battle of Sekigahara in 1600, and thereafter received 1,000 koku as an assistant of Kanetsugu.
後に上杉景勝とその重臣直江兼続の知遇を得て、景勝が越後国から会津藩120万石に移封された1598年から関ヶ原の戦いが起こった1600年のまでの間に上杉氏に仕官し、兼続の与力として1000石を受けた。