Anseong was an area of contention among the three kingdoms of Goguryeo, Baekje, and Silla. It belonged to Baekje before the 4th century, but, in the 5th century, it was taken by Goguryeo as a result of the southward advancement policy of King Gwanggaeto and King Jangsuwang. At that time, Anseong was Naehyehol, Yangseong was Sabokhol, and Juksan was Gaechasangun. In the 6th century, Silla advanced to the Hangang River Basin, and Anseong was incorporated into Silla. As such, Anseong underwent changes after the power struggles of the three kingdoms.
Unified Silla Period
The country was divided into 9 Ju's, leading Anseong to belong to Han-ju. During the reign of King Gyeongdeok, it became one of Hyeon's Baekseong-gun, and Juksan became Gaesan-gun.
In the early Goryeo period, Anseong became Anseong-hyeon; in 1018 (the 9th year of King Hyeongjong's reign), it belonged to Suju (currently known as Suwon); and during King Myeongjong’s reign (1171~1197), it installed an auditor.
In 1413 (the 13th year of King Taejong's reign), the local government system was reorganized, dividing the country into 8 Do's, when Anseong was transferred to Gyeonggi-do from Chungcheong-do. Yangseong was incorporated into Gyeonggi-do after installing a governor, then, in 1895 (the 32nd year of King Gojong's reign), it became Yangseong-gun after the reorganization of the local government system. Juksan was called Juksan in 1413 (the 13th year of King Taejong's reign), and it installed a governor. In 1434 (the 16th year of King Sejong's reign), it was transferred from Chungcheong-do to Gyeonggi-do. In 1543 (the 38th year of King Jungjong's reign), it was promoted to Juksan-bu, along with the installation of a governor. In 1895 (the 32nd year of King Gojong's reign), it became Juksan-gun.
Early Modern Period
In 1914, based on the ordinance of the Governor-General of Korea, the three Guns of Anseong, Juksan, and Yangseong were integrated, becoming Anseong-gun. Anseong-eup became a Eup on July 1, 1937. At that time, in Gyeonggi-do, three administrative districts, namely, Suwon, Anseong, and Gaeseong, became Eups. On January 1, 1963, the administrative districts were reorganized; thus, Gosam-myeon in Yongin-gun was incorporated into Anseong-gun, which thus had 1 Eup, 12 Myeons, and 193 Ris. Due to the administrative reorganization done on February 15, 1983, parts of Gongdo-myeon and Wongok-myeon were incorporated into Pyeongtaek-gun, leading Anseong to have 1 Eup, 12 Myeons, and 188 Ris. On January 1, 1987, parts of Yangbok-ri in Bogae-myeon was incorporated into Geumgwang-myeon, while Nampung-ri, Dongpyeong-ri and Gahyeon-ri in Samjuk-myeon were incorporated into Bogae-meyon. On October 1, 1992, Ijuk-myeon was renamed Juksan-myeon, and, on April 8, 1993, parts of Geonji-ri in Daedeok-myeon and parts of Sohyeon-ri in Daedeok-myeon were incorported into Anseong-eup. On June 6, 1997, parts of Mosan-ri, Geonji-ri, and Sohyeon-ri in Daedeok-myeon, and parts of Gasa-ri and Gahyeon-ri in Bogae-myeon were incorporated into Anseong-eup. On April 1, 1998, Anseong became a city (through Law No. 5458), which thus governed 3 Dongs and 12 Myeons. On June 1, 2001, with the promotion of Gongdo-myeon to Gongdo-eup, Anseong has started governing 1 Eup, 11 Myeons, and 3 Dongs.