Atsuji Sadahiro served as a retainer for the Azai and Oda clans.

In 1570, Sadahiro joined with his father, Atsuji Sadayuki, in the Battle of Anegawa against the Oda.  He was in the third battalion after those led by Isono Kazumasa and Azai Masazumi.  Prior to decimation of the Azai clan, Sadahiro is referenced in a letter of commendation from his lord, Azai Nagamasa. 

Following the loss at Anegawa, Sadahiro and Sadayuki defended Yamamotoyama Castle from an attack by Oda Nobunaga.  In 1573, Hashiba Hideyoshi convinced them to finally surrender to Nobunaga and allow the Oda army into the castle.  This enabled the Oda to surround Odani Castle, defended by Azai Hisamasa. Meanwhile, Nobunaga successfully blocked the Hokkoku kaidō, or northern route, preventing Asakura Yoshikage and a large army en route from Echizen Province from reaching the castle, causing them to take positions in Yogo and  Kinomoto. 

Amid these developments, Asami Michinishi, a retainer of the Azai, surrendered Yakeo fortress, located at the northern base of Ōtake fortress on the same ridge as Odani Castle, further strengthening Nobunaga’s siege of Odani.  Soon after the fall of the castle, Sadahiro and Sadayuki served as leaders in 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, Sadahiro and Sadayuki became yoriki, or local police and retainers, of Hideyoshi, who was based in Nagahama Castle.

In 1575, Sadahiro joined Sadayuki in support of 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 Sadahiro and Sadayuki remained in Ōmi and became hatamoto, or direct retainers, of Nobunaga.  In 1578, Sadahiro and Sadayuki commingled with the umamawari, or mounted soldiers, while assisting Nobunaga in the conduct of a sumō event at Azuchi Castle.  A large and powerful man, Sadahiro competed against Nagata Kagehiro, who was noted as the dominant wrestler at the tournament, but lost.

In 1582, upon learning of the dramatic coup d’état orchestrated by Akechi Mitsuhide against Nobunaga known as the Honnō Temple Incident, Sadahiro and Sadayuki promptly occupied Hideyoshi’s base at Nagahama Castle.  However, after Mitsuhide lost soon thereafter at the Battle of Yamazaki, Sadahiro and Sadayuki were taken prisoner by Hideyoshi and the entire family sentenced to death by crucifixion.