Suganuma Sadatoshi served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He served as the first lord of the Kōzuke-Yoshii domain.  Sadatoshi governed the Ina District in Shinano Province in the era beginning in 1582 when the Tokugawa expanded their territory to encompass Suruga, Kai, and southern Shinano in addition to Mikawa and Tōtōmi provinces.

Sadatoshi was born as the son of Suganuma Sadanao, a member of the Damine-Suganuma clan serving as the main branch of the Suganuma clan based in the Shitara District of Mikawa Province.

In 1573, after Suganuma Sadatada (his cousin and head of the Damine-Suganuma clan) aligned with Takeda Shingen, Sadatoshi and his uncle, Suganuma Sadauji, opposed the action and defected, relying upon Suganuma Sadamitsu of the Noda-Suganuma clan to serve Tokugawa Ieyasu.  In 1575, after the Battle of Nagashino, Sadatada went into exile in the Ina District of Shinano and continued to fight on the side of the Takeda while Sadatoshi remained in the camp of the Tokugawa.  In the third month of 1582, the Takeda clan was extinguished during the Kōshū Expedition led by Oda Nobunaga.  At this time, Sadatada was pursued and killed by the Oda army so Sadatoshi was appointed as his successor to the Damine-Suganuma clan.

Governing the Ina District of Shinano Province

Following the demise of the Takeda clan and after the Tenshō Jingo Conflict, the Ina District formerly held by Sadatada became the territory of the Tokugawa clan.  Under the Tokugawa, governance of the district was allocated among kunishū, or provincial landowners, including Chiku Yoriuji, Hoshina Masanao, Shimojō Yoriyasu, and Kiso Yoshimasa.  In the fourth month of 1584, at the Battle of Komaki-Nagakute, Kiso Yoshimasa defected to the Hashiba clan who were opposed to the Tokugawa.  As a result, the Tokugawa delegated governance of the Ina District to Sadatoshi of the Damine-Suganuma clan owing to his ties to the area.

In the eighth month, Sadatoshi seized Minowa (the territory of the Kiso) and a portion of the territory of the Chiku family in the Ina District.  He then constructed Chikudaira Castle to serve as a base for governing the district.  The next month, he led other landowners in Shinano including Suwa Yoritada and Hoshina Masanao to attack the Kiso clan, only to be repelled by Yamamura Yoshikatsu of Tsumago Castle.  In the eleventh month, owing to suspicions of collusion with the Hashiba, Chikua Yoriuji took his own life.  Thereafter, the territory of the Chiku clan and their band of retainers came under the jurisdiction of Sadatoshi.  In 1587, after the fall of Shimojō Yasunaga as a result of an internal conflict, Sadatoshi moved to Iida Castle formerly occupied by the Shimojō clan.

Sadatoshi exercised autonomy with respect to the governance of his territory in the Ina District.  He strove to advise the landowners aligned with the Tokugawa in his district, serving as their military commander and guaranteeing their freedom of movement.  Ieyasu and Uesugi Kagekatsu were in conflict over the territory of Shinano.  After Kagekatsu came under the command of Hideyoshi, the issue of provincial borders became the subject of discussion whereupon Sadatoshi engaged in negotiations at his base at Iida Castle with Suda Mitsuchika (a retainer of the Uesugi) who was dispatched from Kaizu Castle.

Transfer to the Kantō

After the Conquest of Odawara, when Ieyasu moved to the Kantō, Sadatoshi received a fief of 20,000 koku.  Having the presence of an official representative of the Ina District, Sadatoshi, along with Suwa Yoritada of the Suwa District and Yoda Yasukatsu of the Saku District, were transferred to Kōzuke Province to serve as defenses for the Tokugawa clan in the direction of Shinano.  Sadatoshi conducted a survey of his territory and endeavored to solidify his governance including the holding of Uposatha (Buddhist day of observance) on a regular schedule of six days per month.  In 1600, at the Battle of Sekigahara, Sadatoshi served in the army of Tokugawa Hidetada during an assault on Ueda Castle in Shinano defended by Sanada Masayuki.

In 1602, Sadatoshi died and the bloodline of the Damine-Suganuma clan came to an end, but after his death, the headship of the clan was inherited by Okudaira Tadamasa, a designated heir adopted from his allies, the Okudaira clan.

Sadatoshi was buried at the Jinsō Temple in Yoshii as a family temple, but 150 years later, Suganuma 定用, the third-generation landlord of Shinshiro in Mikawa Province moved Sadatoshi’s monument to the Gentai Temple.  Currently, this monument is designated a historical artifact of the city of Takasaki.