[1] During the time of Silla Kingdom, this region is the administrative center of Saengseoryang-gun (생서량군,生西良郡), which governed the area of today's Seosaeng-myeon, Onsan-eup and Onyang-eup.

In 757, Saengseoryang-gun was renamed Dongan-gun (동안군,東安郡), and Upung-hyeon (우풍현,虞風縣, present day Ungchon-myeon and Yangsan-si's Ungsang region) was placed under Dongan as a subsidiary county.

In the early Joseon dynasty, a naval manhoyeong (군만호영,萬戶營, division with 10,000 soldiers) stationed in Seosaengpo (서생포,西生浦) in the eastern part of the region.

Following the Japanese invasions of Korea in 1592, the commander of the manhoyeong was promoted to the rank of naval dongcheom Jeoljesa (동첨절제사, 同僉節制使).

In the 32nd year of King Gojong's reign (1895), as the navy was being abolished, the region was organized into Seosaeng-myeon and put under the direct jurisdiction of dongcheom Jeoljesa.