Sadakatsu was born in the first month of the third year of Eiroku (1560), the fourth son of Hisamatsu Toshikatsu, in Sakabe Castle [ja], Owari Province (modern-day Agui, Aichi Prefecture).

In the twelfth year of Tenshō (1584), Sadakatsu was the first to arrive on the scene during the Siege of Kanie, part of the Battle of Komaki and Nagakute.

Following the Battle of Nagashino (in which the Tokugawa were victorious over the Takeda), Ieyasu decreed that the daughter of Nobumasa be introduced as the wife of Sadakatsu.

Upon their marriage, Sadakatsu's wife's younger brother, Okudaira Sadayoshi, and uncle were transferred into the new family.

In the ninth month of the eighteenth year of Tenshō (1590), Sadakatsu was granted a 3,000-koku domain in Shimōsa Province, near modern-day Tōnoshō, Chiba Prefecture.

In the third month of the seventh year of Keichō (1602), Tokugawa Ieyasu's tenth son (eventually named Yorinobu) was born in Fushimi Castle, Yamashiro Province.

In the fifth month of the tenth year of Keichō (1605), Sadakatsu's daughter Kuma-hime was adopted by Ieyasu and married to Yamauchi Tadayoshi [ja].