Ōta Sukemasa served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He served as the lord of Iwatsuki Castle in the Saitama District of Musashi Province and, later, as the lord of Katano Castle in Hitachi Province.

In 1522, Sukemasa was born as the second son of Ōta Sukeyori, a retainer of the Ōgigayatsu-Uesugi clan.

Initially, Sukemasa followed his father, Sukeyori, and older brother, Ōta Sukeaki, in service of the Ōgigayatsu-Uesugi clan.  In 1536, after the death of Sukeyori, Sukeaki inherited the headship of the clan, but Sukemasa did not get along well with Sukeaki so he left Iwatsuki Castle and resided with his father-in-law, Nanbada Norishige, at Matsuyama Castle in the Yokomi District of Musashi.  Thereafter, Sukemasa favored the submission by Sukeaki to the Hōjō clan of Sagami Province, but, together with Norishige, continued in service of the Ōgigayatsu-Uesugi.  In 1537, in battle against the Hōjō, Norishige lost three sons and a nephew named Nanbada Hayato-no-shō so Sukemasa, as the adopted son-in-law of Norishige, became the designated successor to Norishige at Matsuyama Castle.

In 1546, his lord, Uesugi Tomosada, died in final showdown against Hōjō Ujiyasu during the Siege of Kawagoe Castle.  This threatened the continued existence of the Ōgigayatsu-Uesugi clan.  Sukemasa departed from Matsuyama Castle and lived in reduced circumstances under the control of the Yokose clan in Nitta in Kōzuke Province.  In the ninth month of 1547, Sukemasa took advantage of an opening with the Hōjō clan to launch a surprise attack to recapture Matsuyama Castle.  In the tenth month, his older brother, Sukeaki, died, leaving Iwatsuki Castle absent a lord. In the twelfth month, he assaulted Iwatsuki Castle, thereby forcibly taking over the headship of the clan, although some of the retainers who supported the Hōjō absconded for the protection of the Hōjō.  Before long, the Hōjō faction rolled back and Ueda Tomonao (to whom Matsuyama Castle was entrusted) switched sides to the Hōjō.  After the surrounding of Iwatsuki Castle in the first month of 1548, Sukemasa surrendered to the Hōjō.  In the fourth month of 1554, he adopted the informal title of Mino-no-kami which had been used by his father.

Around this time, the Hōjō clan of Sagami Province entered into a three-way alliance with the Takeda clan of Kai and the Imagawa clan of Suruga.  In the northern Kantō, a tensions arose between the Hōjō and the Uesugi clan of Echigo Province.  As a retainer of the Hōjō, Sukemasa fought primarily in the direction of Hitachi Province while serving on behalf of the Hōjō as an intermediary with other clans including the Yokose, the Shirakawa-Yūki, and the Date.  Considering the high reputation of the Ōta family from which Sukemasa descended, Hōjō Ujiyasu permitted Sukemasa to serve as a nominal retainer of Ashikaga Yoshiuji, the Koga kubō.  Further, he arranged for the engagement of his daughter to Sukemasa’s lineal heir, Ōta Ujisuke.

In 1560, Uesugi Kenshin of Echigo led a large army to invade the Kantō, an event known as the Siege of Odawara Castle.  In the twelfth month, Sukemasa responded by serving in the vanguard of the Uesugi army, making clear his abandonment of the Hōjō in favor of the opposing Uesugi.  Seeking retribution against Sukemasa, Ujiyasu ordered repeated attacks against Iwatsuki and Matsuyama castles in Musashi.  During the time that Sukemasa was under the command of the Uesugi, Sukemasa continued to serve as an intermediary with the Satomi and Nasu clans.  On 7/2 of Eiroku 6 (1563), he was invested by the Imperial Court with the title of Vice Minister of Popular Affairs.  There is a theory that this was part of a policy of appeasement by the Hōjō, but Sukemasa’s posture did not change.

In 1564, Sukemasa joined with the allied Satomi clan to battle against the Hōjō at the Second Battle of Kōnodai, but was defeated and fled with the Satomi army for a while to Kazusa Province.  In the fifth month, with the support of Sakai Taneharu of Kazusa, he returned to Iwatsuki Castle.  In the seventh month, when Sukemasa paid a secret visit to the Satomi clan to request their support, his son, Ujisuke (who was an advocate of the Hōjō), incarcerated Kajiwara Masakage (Sukemasa’s younger brother of a different mother) and his mother (a consort of Sukemasa from the Ōishi clan), occupied Iwatsuki Castle, and surrendered to the Hōjō.  This resulted in the ouster of Sukemasa from Iwatsuki Castle.  Sukemasa was then compelled to rely upon his son-in-law, Narita Ujinaga, for aid and aimed with Yokose Narishige (later known as Yura Narishige), in the fifth month of 1565, to recapture Iwatsuki Castle, but the plan failed.  Next, he entered the priesthood, adopting the name of Sanrakusai Nyūdō Dōyo.

Before long, Sukemasa turned to the Shimotsuke-Utsunomiya clan, followed by Satake Yoshishige of Hitachi Province, coming under his command.  After Yoshishige defeated Oda Ujiharu (who was aligned with the Hōjō) and took Katano Castle, he appointed Sukemasa to serve as its lord.  Sukemasa converged with Masakage and his mother who managed to escape their confinement and engaged in battle against the Hōjō from Katano Castle.  Meanwhile, he joined with Makabe Hisamoto to strengthen the power of the Satake clan.  Kakioka Castle was originally held by the Makabe clan, but was transferred to Hisamoto’s son-in-law, Kajiwara Masakage, so, along with Katano Castle, Kakioka served as a base for Sukemasa.  Sukemasa was requested to serve on behalf of the Satake as an intermediary with Uesugi Kenshin and Takeda Shingen.   Kenshin, however, was dissatisfied with Sukemasa’s leaning toward the Satake clan despite originally serving as a retainer of the Uesugi.

In 1569, the three-party alliance collapsed and, after the Hōjō clan came into conflict with the Kai-Takeda, they entered into an alliance with Uesugi Kenshin of Echigo.  This is known as the Echigo-Sagami Alliance.  This alliance between the Hōjō and the Uesugi raised concerns among the kunishū, or provincial landowners, in the Kantō.  Sukemasa was opposed to an alliance with Kenshin.  In the course of deliberations, issues regarding dealings with Sukemasa also arose as an important topic.  After the death in battle of Ujisuke at the Battle of Mifuneyama, the Hōjō and the Uesugi both requested that, rather than assign Sukemasa to Iwatsuki Castle, that Sukemasa should cut ties with the Satake clan and relinquish his landholdings in Hitachi including Katano Castle.  Nevertheless, consensus could not be reached in regard to the disposition of Matsuyama Castle which was the subject of a dispute between Sukemasa and Ueda Tomonao.  Seeking guarantees for his landholdings in Hitachi similar to those for Iwatsuki, Sukemasa and Satake Yoshishige opposed these proposals.  After marching into the Kantō in the tenth month, Kenshin ordered Sukemasa to deploy to Kōzuke, to abandon Katano Castle in Hitachi, and not to return to the service of the Satake.  The next month, the same orders were issued again.  Sukemasa, however, ignored the orders and, in the twelfth month, together with Satake Yoshishige and Makabe Hisamoto, defeated Oda Ujiharu (an ally of the Hōjō) at the Battle of Tebaizaka.  Further, they occupied Oda Castle and placed Ujiharu under the command of Sukemasa.

Sukemasa’s refusal to obey the orders issued by Kenshin, including the deployment to the Kantō and the abandonment of Katano and Kakioka castles in Hitachi, led to a definitive break in their relations.  In the third month of 1570, Kenshin sent a letter to Mino-no-kami (Sukemasa) permanently severing their ties.  Later, Kenshin regretted sending the note, and, via intermediaries including Yamayoshi Toyomori and Kawada Nagachika, he attempted to reconcile with Sukemasa.  By the eighth month of 1572, negotiations between the parties resumed but a final settlement (including with the Satake clan) could not be reached until the eighth month of 1575 after the Oyama clan was attacked by the Hōjō and the Satake and Uesugi clans came to their aid.  During this year, Sukemasa focused his efforts on forging an alliance to restore relations between the clans including with the Satomi.  Following the death of Kenshin on 3/13 of Tenshō 6 (1578), in the wake of the ensuing succession struggle between his adopted sons known as the Otate Conflict, Sukemasa endeavored with Ashikaga Yoshiatsu to foster an alliance between Satomi Yoshiyori and Takeda Katsuyori whose relationship with the Hōjō had deteriorated.  From around this time, Sukemasa maintained communications with Oda Nobunaga and, in 1578, after Sukemasa learned of the death of Kenshin from Uesugi Kagekatsu, Sukemasa informed Nobunaga of the news.

After the decimation of the Kai-Takeda by Oda Nobunaga in the third month of 1582, Sukemasa sought to become an immediate follower of Nobunaga on the basis that he be placed under the command of Takigawa Kazumasu.  He also confirmed via Kazumasu whether it would be possible to install Ashikaga Yoriatsu (Ashikaga Yorizumi) as the Koga kubō in lieu of Ashikaga Yoshiuji who maintained close relations with the Hōjō clan.  On 6/2 of Tenshō 10 (1582), Oda Nobunaga died in a coup d’état launched by Akechi Mitsuhide, an event known as the Honnō Temple Incident.  In the ensuing chaos, divisions between the Hōjō and Satake clans deepened.  The Battle of Numajiri broke out in the fifth month of 1584.  In the sixth month, in the midst of the conflict, Sukemasa’s second son, Kajiwara Masakage, suddenly colluded with the Hōjō and rebelled against the Satake.  After the parties reached a settlement in the seventh month, Satake Yoshishige took steps to eliminate Masakage and, in the tenth month, Masakage surrendered to Yoshishige but his long period of service was disregarded.  Owing to these events, Sukemasa designated his third son, Ōta Suketake, as his successor and, aorund 1588, Suketake inherited the headship of the clan.

In 1590,  by the time of the Conquest of Odawara, Toyotomi Hideyoshi had completely subjugated western Japan and was on the course of becoming the most powerful figure in Japan.  Sukemasa participated in this campaign in Odawara during which he met Hideyoshi.  He did not, however, return to his former home in Iwatsuki and, in 1591, died of illness.

Sukemasa transferred Katano Castle to his third son, Ōta Suketake.  Later, Suketake served Yūki Hideyasu and his descendants became retainers of the Fukui domain.  However, his great-grandchild, Ōta Koresuke, became a rōnin, or wandering samurai, and, by dying before having children, the lineage was extinguished.  Documents regarding the Ōta family owned by Koresuke were transferred to his wife (a daughter of the Jimyō Temple) and subsequently transferred to the Sensō Temple (that had close relations with the Jimyō Temple) where the records remain to the present time.

Anecdote

When Sukemasa resided in Iwatsuki and Matsuyama Castles, he is said to have kept fifty dogs at each castle.  Fond of his dogs, he often spent time with them to the extent some would ask if he was alright.  He brought dogs back and forth between the castles and, on occasion, ordered them released.  When Matsuyama Castle came under attack by Hōjō Ujiyasu, his retainers attached bamboo cylinders to the dogs containing messages for help and released them.  The dogs went straight to Iwatsuki Castle whereupon, after receiving the messages, Sukemasa dispatched reinforcements and expelled the Hōjō forces from Matsuyama Castle.  This episode is noted as the first instance where dogs were used for military purposes in Japan.