Aoki Sadaharu served as a bushō during the Sengoku period. He was a retainer of the Tokugawa clan.
In 1568, Tokugawa Ieyasu invaded the territory of Imagawa Ujizane in Tōtōmi Province, capturing Futamata Castle defended by Matsui Munetsune.
Initially, Ieyasu placed Udono Ujinaga as the chamberlain of the castle. An attack by the Takeda army from Kai Province heightened the risk, so Ieyasu replaced Ujinaga with Nakane Masateru. Sadaharu, along with Matsudaira Yasuyasu, also entered the castle. In the tenth month of 1572, Futamata Castle was attacked by Takeda Shingen in an event known as the Siege of Futamata Castle. After a valiant fight, Sadaharu and the other defenders finally surrendered, vacating the castle. Following his departure, Sadaharu returned with Masateru to Hamamatsu Castle.
On 12/22 of Genki 3 (1572), at the Battle of Mikatagahara, Sadaharu fought with all of his effort in a bid to wipe out the disgrace he felt after the surrender of Futamata Castle but was killed in action.