Nagao Tadakage served as a bushō from the late Muromachi period to the Sengoku period.  Tadakage was the fifth head of the Sōja-Nagao clan of Kōzuke Province and the kasai, or head of house affairs, of the Yamanouchi-Uesugi family.  Tadakage also served as the deputy military governor of Kōzuke and Musashi provinces.

Based on genealogical records, Tadakage was adopted by Nagao Kagemune or Kagemune’s younger brother, Yoshizumi (both of whom died early), but, based on records from the Chōrin Temple, Tadakage inherited the headship of the Yamauchi-Uesugi clan from their father, Nagao Tadamasa.  In a letter from between 1473 and 1476 regarding a dispute with respect to the collection of rent on land owned by Tadakage, Tadakage inherited the land from Tadamasa several years prior.  Tadamasa also served as the head of house affairs.  Based on references in the letter to Hōtoku 2 (1450), Tadakage had already succeeded to the headship of the Sōja-Nagao clan in the Bunan era (1444 to 1448) while Tadamasa was still alive.  Therefore, after the early demise of his two natural sons, Tadamasa appears to have adopted Tadakage.  Thereafter, Tadakage assumed important roles in the Yamanouchi-Uesugi family that had been performed by his adoptive father including as the deputy military governor of Musashi Province.  In 1444, Tadamasa transferred the role of head of house affairs for the Yamanouchi-Uesugi family to Nagao Kagenaka (Tadakage’s natural father) of the Shiroi-Nagao clan.  After Kagenaka, Tadakage’s older brother, Nagao Kagenobu, became the next head of house affairs.

Following the demise in 1450 of his adoptive father, Tadamasa, the selection of Tadakage as the successor to Kagenobu was not unnatural given the prior methodology for filling the position of head of house affairs.  Meanwhile, after having members of the Shiroi-Nagao serve in the role for two successive generations, Uesugi Akisada (the head of the Yamanouchi-Uesugi clan) feared the branch would become too strong, providing another reason to assign the role to Tadakage instead of Kagenobu’s son, Nagao Kageharu (who was Akisada’s nephew).  Therefore, Akisada did not choose Kageharu, choosing to appoint Tadakage as the head of house affairs instead.  This was a factor leading to an event known as the Revolt of Nagao Kageharu in the sixth month of 1476 in which Kageharu opposed Akisada and pledged allegiance to Ashikaga Shigeuji, the Koga kubō.

The Sōja-Nagao clan and the Shiroi-Nagao clan were of equal status.  There is a theory that, when appointing successors to the head of house affairs, the eldest member from among both clans would be chosen on an alternating basis.  Based on this theory, even though Tadakage was the adopted heir of Tadamasa, owing to age, he could not assume the role of head of house affairs for a long time.  Meanwhile, although Kageharu was Kagenobu’s eldest son and heir, he could not assume the role.  Tadakage served as a commander on behalf of Akisada to subdue the rebellion by his nephew, Kageharu, but was unable to prevail against Kageharu in the Irako War, after which Akisada fled to Kōzuke.  Despite having quelled the rebellion, the outbreak of the Chōkyō War turned a formerly cooperative relationship into one of confrontation between the Yamanouchi and Ōgigayatsu families of the Uesugi clan.  In addition, Tadakage’s home province of Kōzuke witnessed the Jōshū ikki, or uprisings, by kokujin, or provincial landowners, including the Nagano clan so that Tadakage continued to lead a life enveloped in conflict.

Tadakage is recognized for redeveloping the Unchō hermitage at the Engaku Temple in Kamakura.