Saionji Sanemitsu served as a daimyō and head of the Iyo-Saionji clan based in the southern portion of Iyo Province in Shikoku.

In 1510, Sanemitsu was the son of Saionji Kinie (under an alternate theory, he was the son of Kinie’s father, Saionji Kinsue).  The Saionji wielded power in the southern portions of Iyo.  Sanemitsu moved from Matsuba Castle to Kurose Castle (both located in mountainous terrain) to be in a better position to defend against the Ichijō clan from neighboring Tosa Province and the Ōtomo clan from Bungo Province in Kyūshū.

In 1556, Sanemitsu engaged in territorial battle against Utsunomiya Toyotsuna based in nearby Ōzu Castle.  This clash resulted in a loss and the death of his son and designated heir, Saionji Kintaka. Sanemitsu then settled with the Utsunomiya through the mediation of Kōno Michinobu.  In place of his natural son, Sanemitsu adopted his nephew, Saionji Kinhiro, to become his newly designated heir.  Kinhiro then married Sanemitsu’s daughter, Nishihime.

In the summer of 1560, Sanemitsu was awarded the titles of Junior Fifth Rank (Lower), and Major General of Imperial Guards of the Left Division.  In the spring of 1565, Sanemitsu traveled to the capital of Kyōto to participate in a waka (classical poetry) event at the residence of Saionji Kintomo of the main branch of the Saionji clan.  He died later that year at the age of fifty-six.  Sanemitsu was succeeded by Kinhiro, who became the eighth and final head of the Iyo-Saionji clan.