Born in Mikawa Province as the second son of Torii Mototada, a senior hereditary retainer of the Tokugawa. He inherited the family headship in 1600 after his father died defending Fushimi Castle in the prelude to Sekigahara. Following the battle, in recognition of his father's service, Tadamasa was granted four districts of southeastern Mutsu Province (100,000 koku) and entered Odate Castle, previously the seat of the Iwaki clan, renaming the area from "Iwaki" to "Iwakitaira." Construction of Iwakitaira Castle began in 1603 and was completed in 1615 after twelve years. In 1622, following the dispossession of the Mogami clan, he was transferred with an increased stipend to Yamagata (220,000 koku) in Dewa Province, becoming its first domain lord. As a hereditary Tokugawa vassal he served as a watchdog over the Tohoku daimyo, and died in Yamagata in 1628.

Inheriting the loyalty of his father who died at Fushimi Castle, he was a capable administrator entrusted with strategic posts in Tohoku as a hereditary Tokugawa daimyo, known also for his twelve-year dedication to completing Iwakitaira Castle.

As the builder of Iwakitaira Castle, he laid the urban foundations of what is now central Iwaki City. Serving as first lord of both Iwakitaira and Yamagata domains, he contributed to establishing Tokugawa control over the Tohoku region.