Naitō Ienaga served as a bushō during the Sengoku and Azuchi-Momoyama periods.  He was a retainer of the Tokugawa clan.  He had the nickname of Kinichirō.

The Naitō clan of Mikawa served as hereditary retainers of the Matsudaira clan (later the Tokugawa clan) from the era of his grandfather.  Ienaga was born as the son of Naitō Kiyonaga in the latter years of Kiyonaga’s life.  Ienaga served under Tokugawa Ieyasu and succeeded his older brother-in-law, Naitō Nobunari, as the head of the Naitō family.

Similar to his father, Ienaga demonstrated valor on the battlefield and was an expert at archery, referred to as the “greatest bow hand.”  Also like his father, Ienaga was an adherent of the Ikkō sect, but, during the Mikawa Ikkō-ikki, Ienaga differed from his father, choosing not to join the ikki forces.  Instead, he obeyed Tokugawa Ieyasu (his uncle through family marriage) and showed his loyalty by suppressing the ikki forces.  This earned him the trust of Ieyasu.

He was conferred one of the characters from the name of Ieyasu and adopted the name of Ienaga.

On 8/9 of Tenshō 18 (1590), Ienaga was granted 12,000 koku in addition to his former fief of 8,000 koku, yielding a total of 20,000 koku.  He also became the lord of Sanuki Castle in Kazusa Province.

In 1600, Ienaga, along with Torii Mototada and Matsudaira Ietada, defended Fushimi Castle, provoking a revolt by Ishida Mitsunari as the leader of the Western Army.  The Western Army raised arms, but Ienaga, along with Torii Mototada and Matsudaira Ietada, were killed in action during the Siege of Fushimi Castle occurring as a preliminary contest leading to the Battle of Sekigahara on 9/15 of Keichō 5 (1600).  He was fifty-five years old.  Ienaga was succeeded by his eldest son, Naitō Masanaga.  As recognition for Ienaga’s contributions in battle, Masanaga received an increase of 10,000 koku to his fief.  Masanaga later became a daimyō and the head of the Kazusa-Sanuki domain and the Mutsu-Iwakitaira domain.

Ienaga was interred at the Dairen Temple in Miidera in Ōtsu in Ōmi Province and later moved to the Zenshō Temple in Hanagayatsu in Sanuki in Kazusa Province.