During the Three Han Period, the present-day Dangjin was part of the territory of the Mahan confederacy, while in the Three Kingdoms Period, Baekje governed over a large area of Dangjin, with Myeoncheon-myeon referred to as Hye-gun at the time. Beolsuji-hyeon referred to the area of Dangjin and Godae-myeon, while Jeongmi-myeon belonged to Yeochon-hyeon. Sapyeong-hyeon, which was the area centering on Sinpyeong, governed over the large coastal region from Songak Hanjin.
During the reign of Gyeongdeok, the 35th king of Silla, Hye-gun was renamed as Hyeseong-gun, Beolsuji-hyeon as Dangjin-hyeon, Sapyeong-hyeon as Sinpyeong-hyeon, and Yeochon-hyeon as Yeoeup-hyeon.
In the early Goryeo Dynasty, Dangjin was governed by a powerful local family, and later in 983 (2nd year of King Seongjong’s reign), it became a part of Gongju when the Korean peninsula was divided into 12 provinces. In 1018 (9th year of King Hyeonjong’s reign), Dangjin was called Dangjin-hyeon, and it was a part of Unju (present-day Hongseong), and in 1105 (1st year of King Yejong’s reign), a Gammu was appointed. In 1358 (7th year of King Gongmin’s reign) near the end of the Goryeo Dynasty, Dangjin belonged to Hongju-mok.
In the early Joseon period, Dangjin belonged to Hongju-mok, as it did in the late Goryeo period. Then, in 1414 (13th year of King Taejong’s reign), it became Dangjin-hyeon, with a Hyeongam appointed to govern over the area. In the late Joseon period in 1895 (32nd year of King Gojong’s reign), it became Dangjin-gun, with eight myeons including Gunnae, Dong-myeon, Nam-myeon, Sangdae, Hadae, Gosan, Oemaeng and Naemaeng. Then, during the reorganization of guns and myeons in 1914, the twenty-two myeons of Myeoncheon-gun, two ris of Daeunsan Mountain in Ildo-myeon, Haemi-gun, part of Samgok-ri, Yeomsol-myeon, and two ris (Okgeum-ri and Heukseok-ri) of Sinjong-myeon, Yesan-gun were incorporated into Dangjin-gun. Thus, Dangjin was reorganized to include 123 ris and 10 myeons (Ibae, Godae, Seongmun, Maam, Songsan, Sunseong, Beomcheon, Hapdeok, Songak and Sinpyeong). Later, Maam-myeon was renamed as Myeoncheon-myeon in October 1917, Ibae-myeon as Dangjin-myeon in 1928, and Beomcheon-myeon as Ugang-myeon on October 1, 1942. Then, on November 6, 1957, and 18 ris of Jeongmi-myeon and Daehoji-myeon of Seosan-gun were incorporated into Dangjin-gun, in accordance with Act No. 456, and it came to have a total of 12 myeons under its jurisdiction.
Dangjin-myeon was promoted to Dangjin-eup, in accordance with Act N. 1177, on January 1, 1963. Hapdeok-myeon was promoted to Hapdeok-eup on July 1, 1973. Also, Yeomi-ri, Jeongmi-myeon was incorporated into Unsan-yeon, Seosan-si, and thus Dangjin came to be comprised of 149 ris, 10 myeons and 2 eups.
Songak-myeon was promoted to Songak-eup on January 1, 2010, and thus Dangjin came to be comprised of 149 ris, 9 myeons and 3 eups.
On January 1, 2012, Dangjin-gun (Dangjin County) became Dangjin-si (Dangjin City), with 3 dongs, 9 myeons and 2 eups (138 ris and 11 dongs). Dangjin is located in the northern part of Chungcheongnam-do Province, facing Asan-si and Pyeongtaek-si of Gyeonggi-do Province toward the east, Seosan-si toward the west, Yesan-gun and Seosan-si toward the south, and Hwaseong-gun, Gyeonggi-do toward the north. It is generally comprised of large plains that are connected to mountains and hills, and there has been balanced growth and development between the agricultural and steel industries.