Atsuji Sadayuki served as a retainer of the Azai and the Oda. He was lord of Yamamotoyama Castle in Ōmi Province.
Sadayuki was from a family of kokujin status for generations in the Ika District of northern Ōmi. He supported the Azai when the clan expanded their influence in place of the Kyōgoku. As a senior retainer of the Azai, Sadayuki was assigned the stronghold of Yamamotoyama Castle overlooking the northern portion of Lake Biwa and the Hokkoku kaidō, or northern route, which ran in a North-South direction along the eastern side of the lake.
In 1570, Sadayuki led a battalion of 1,000 mounted soldiers in the Battle of Anegawa, serving as a commander of the third battalion after those led by Isono Kazumasa and Azai Masazumi. He participated in other battles against the Oda and defended Yamamotoyama Castle from an attack by Oda Nobunaga. In 1573, Hashiba Hideyoshi convinced Sadayuki to secretly collaborate with Nobunaga and allow the Oda army into Yamamotoyama Castle, isolating Odani Castle under control of Azai Hisamasa. After surrendering to the Oda clan, Sadayuki and his son, Atsuji Sadahiro, soon led the attack against Asakura Yoshikage at the Battle of Ichijōdani Castle.
After defeat of the Azai, Nobunaga awarded control of northern Ōmi to Hashiba Hideyoshi, and with the capture of Yamamotoyama Castle and pacification of the Ika District, Sadayuki and Sadahiro became yoriki, or local police and retainers, of Hideyoshi.
In 1575, Sadayuki and Sadahiro supported Hideyoshi’s campaign against the uprisings by the Ikkō sect in Echizen Province. Meanwhile, Sadahiro appealed in a letter to Sugaya Nagayori, a senior retainer of Nobunaga, after Hideyoshi became increasingly oppressive and took away the majority of their stipend from Chikubu Island, located in the northern area of Lake Biwa. Beginning in 1577, Hideyoshi headed toward Harima Province in connection with the broader objective to conquer the provinces in the Chūgoku Region, while Sadayuki and Sadahiro remained in Ōmi and became hatamoto, or direct retainers, of Nobunaga. In 1578, Sadayuki and Sadahiro commingled with the umamawari, or mounted soldiers, while assisting Nobunaga in the conduct of a sumō event at Azuchi Castle.
Thereafter, Sadayuki defended Kinometōge Castle in Echizen, but it was toppled in an attack by followers of the Hongan Temple.
In 1582, upon learning of the dramatic coup d’état orchestrated by Akechi Mitsuhide against Nobunaga known as the Honnō Temple Incident, Sadayuki and Sadahiro promptly occupied Hideyoshi’s base at Nagahama Castle. However, after Mitsuhide lost soon thereafter at the Battle of Yamazaki, Sadayuki and Sadahiro were taken prisoner by Hideyoshi and the entire family sentenced to death by crucifixion.