Nagakura Sukemasa served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a retainer of the Hyūga-Itō clan.

The Nagakura were an illegimate branch of the Itō clan, founded by Nagakura Sukeuji, the son of Itō Suketoki, the first-generation head of the Itō from the early Kamakura period.  When Itō Ujisuke moved to Hyūga Province, members of the Nagakura accompanied him and became servants of the Itō.

In 1567, after Sukemasa killed Aokage Kageyu of the opposing Shimazu army, he was rewarded by Itō Yoshisuke by receiving the Itō surname and adopted the name of Itō Kageyu.  Later, he reverted to the surname of Nagakura and had his lineal heir, Nagakura Ōmi-no-kami, adopt the Itō surname.  In 1572, after the Itō suffered a defeat at the Battle of Kizakibaru, upon orders of Yoshisuke, Sukemasa defended Takaharu Castle on the front lines.  During festivals at the Kirishima Shrine in Hyūga, Sukemasa invaded a village in the territory of the Shimazu, shuttering the events.  In 1576, he was attacked by the Shimazu and vacated Takaharu Castle.

In 1577, Sukemasa, accompanied by his lineal heir, Ōmi-no-kami, and his second son, Rokurō-tarō, joined the withdrawal by the Itō clan to Bungo Province.  In 1578, in a bid to recover the Itō territory, he launched a rebellion at Sannō in Hyūga.  He then attacked Aya Castle and, after gathering members of the Itō band of retainers from the Forty-Eight Castles of the Itō, proceeded to attack Tonokōri Castle, but the effort failed.  In the ninth month, the Ōtomo army headed south toward Hyūga.  Sukemasa, along with Yamada Munemasa and other retainers of the Itō, waged a defense of Niiro-Ishino Castle against Shimazu forces.  In the end, the Shimazu were repelled.  This event is known as the Siege of Ishino Castle.

At the Battle of Mimikawa, Sukemasa, along with his second son, Rokurō-tarō, took their own lives.  The whereabouts of his lineal heir, Ōmi-no-kami, are unknown.