He rose to become considered one of Ieyasu's sixteen generals and was entrusted with Futamata Castle in Tōtōmi Province.

Tadayo contributed in the Battle of Azukizaka (1564) against the Ikkō sect in Mikawa province.

At the Battle of Mikatagahara on 1573, Ōkubo Tadayo along with Amano Yasukage led a small band of Tokugawa foot soldiers and matchlock gunners in attacking the Takeda camp,[1] throwing the vanguard of the Takeda army into confusion.

[3] Upon the assassination of Oda Nobunaga in 1582, Ieyasu expanded his rule into Shinano Province, with Ōkubo Tadayo assigned to managing the campaign from his base at Komoro Castle.

Toyotomi Hideyoshi ordered that Ōkubo Tadayo be raised at that point to the status of daimyō, and was assigned the fief of Odawara, with an income of 45,000 koku.