Goryeo had been a semi-autonomous vassal state under the overlordship of the Mongol Yuan dynasty since the Mongol invasions of Korea in the 13th century. Starting with King Chungnyeol, prospective rulers of Korea married Mongolian princesses and were customarily sent to the Yuan Court, in effect, as hostages. As per this custom, King Gongmin spent many years in the Yuan court, being sent there in 1341, before ascending the Korean throne. He married a Mongolian princess who became Queen Noguk. The Yuan dynasty began to crumble during the mid-14th century, and was eventually conquered and replaced by the Ming dynasty in 1368.
With the disintegration of Yuan, which had influenced the Korean peninsula since the Mongol invasion of Korea of 1238, King Gongmin began efforts to reform Goryeo government. His first act was to remove all pro-Mongol aristocrats and military officers from their positions. These deposed people formed a dissident faction which plotted an unsuccessful coup against the king. High official Jo Il-shin even tried to take over the government, but this rebellion was put down by general Choi Young.
During the Mongol invasions of Korea, between the 1250s and the 1270s, the Mongols had annexed the northern provinces of Korea and incorporated them into their empire as Ssangseong (쌍성총관부, 雙城總管府) and Dongnyeong Prefectures (동녕부, 東寧府). In 1356, the Goryeo army retook these provinces partly thanks to the defection of Yi Ja-chun, a minor Korean official in the service of the Mongols in Ssangseong, and his son, Yi Seong-gye. In addition, Generals Yi Seong-gye and Ji Yongsu led a campaign into Liaoyang.
Another issue was the question of land holdings. The land-grant system had broken down, and Mongol-favoured officials, along with a handful of landed gentry, owned the vast majority of agricultural land, which was worked by tenant farmers and bondsmen. However, King Gongmin's attempt at land reform was met with opposition and subterfuge from those officials who were supposed to implement his reforms, as they were landowners themselves.
The Wokou were also a problem encountered during King Gongmin's reign. The Wokou had been troubling the peninsula for some time and had become well-organized military marauders raiding deep into the country, rather than the "hit-and-run" bandits they started as. Generals Choi Young and Yi Seong-gye were called upon by King Gongmin to combat them.
Additionally, King Gongmin grappled with the Red Turban troops, who invaded Goryeo twice during his reign (first in 1359 and again in 1361). In 1361, the Red Turban troops occupied Kaesong for a short period of time. After Kaesong was recaptured by Generals Choe Yeong, Yi Seong-gye, Jeong Seun, and Yi Bang-sil, few Red Turban troops managed to escape with their lives.
During the reign of King Gongmin, a Goryeo diplomat, Mun Ik-jeom, stationed in China managed to smuggle cotton seeds into Goryeo, introducing them to the Korean peninsula for the first time.
Although the relationship between Queen Noguk and the king was very close, they failed to conceive an heir for many years. Despite suggestions of taking a second wife, King Gongmin ignored these requests. The king was also known to have entered into pederastic relationships with several court catamites, or chajewi, and the names of five of these are recorded as: Hong Yun, Han An, Kwon Chin, Hong Kwan, and No Son. Queen Noguk became pregnant but died from complications with childbirth in 1365. Her death led to King Gongmin's depression and mental instability. King Gongmin became indifferent to politics and entrusted the great tasks of state to Pyeonjo, a Buddhist monk who was born as the son of a princess and a slave. Judging him as clever, King Gongmin renamed Pyeonjo as Shin Don. Having the full confidence of King Gongmin, Shin Don tried to reform the society of Goryeo. In 1365, King Gongmin gave Pyeonjo the nickname "Cheonghan Geosa" and the noble title of Jinpyeonghu (Chinpyŏng Marquess). After six years, Shin Don lost his position and King Gongmin had him executed in 1371.
Goryeo's entrenched bureaucracy never forgave King Gongmin for his reform efforts. They interpreted his policy of cutting all ties with the Yuan and establishing relations with Ming China as a direct threat to their status and feared that further attempts at reform might yet be made. Kaesong's deposed pro-Mongol faction battled to protect its position and hoped to renew ties with the Mongols who had helped them gain and hold their wealth in the first place.
In 1374, he was killed by Choe Man-saeng (최만생) and others. One of the young men, Hong Ryun (홍륜) had relations with one of King Gongmin's concubines, which led to King Gongmin's anger. So before King Gongmin could kill him, Hong Ryun and Choe Man-saeng killed King Gongmin in his sleep.
After his death, a high official Yi In-im assumed the helm of the government and enthroned eleven-year-old, King U.
Although he did not receive a temple name of a king, because the political situation of the time following his death did not recognize him as such, he proclaimed himself king as a part of reformations he undertook in order to reinstate Goryeo's position as an independent nation.
King Gongmin was well known for his artistic skills, and he is referred to as one of the best artists of the Goryeo period. He was also well known for his calligraphy works.
Example of his works are:"Painting of A Hunt in the Mountains of Heaven" 《天山大獵圖 (천산대렵도 Cheonsan Daeryeop Do)》
"Painting of Two Sheep" 《二羊圖 (이양도 I Yang Do)》
"Portrait of Princess Noguk" 《魯國大長公主真 (노국대장공주진 Noguk Daejang Gongju Jin)》
"Portrait of Yeom Je-shin 《廉悌臣象 (염제신상 Yeom Je-shin Sang)》, 1370's
"Portrait of Sohn Hong-ryang" 《孫洪亮象 (손홍량상 Sohn Hong-ryang Sang)》
"Portrait of Śākyamuni's Leaving Mountain" 《釋迦出山像 (석가출산상 Seokga Chulsan Sang)》
"Landscape of Epang Palace" 《阿房宮圖 (아방궁도 Ahbanggung Do)》
"Landscape of Hyeonreung" 《玄陵山水圖 (현릉산수도 Hyeonreung Sansu Do)》
"Portrait of Bodhidharma Crossing a River with a Broken Branch" 《達磨折蘆渡江圖 (달마절로도강도 Dalma Jeollo Dogang Do)》
Dongjabohyeon Yugabaeksang Do《童子普賢六牙白象圖 (동자보현육아백상도)》
Father: King Chungsuk of Goryeo (30 July 1294 – 3 May 1339) (고려 충숙왕)
Grandfather : King Chungseon of Goryeo (20 October 1275 – 23 June 1325) (고려 충선왕)
Grandmother : Royal Consort Yasokjin Ui-Bi (? – 18 July 1316) (의비 야속진)
Mother: Queen Gongwon of the Namyang Hong clan (18 July 1298 – January 1380) (공원왕후 홍씨)Grandfather : Hong Gyu (1242 – 23 June 1316) (홍규)
Grandmother : Lady Kim of the Gwangju Kim clan (광주 김씨)
Portrayed by Im Hyuk in the 1983 KBS TV series Foundation of the Kingdom.
Portrayed by Jeong Bo-seok in the 2005–2006 MBC TV series Shin Don.
Portrayed by Joo Jin-mo in the 2008 film A Frozen Flower.
Portrayed by Ryu Deok-hwan in the 2012 SBS TV series Faith.
Portrayed by Ryu Tae-joon in the 2012–2013 SBS TV series The Great Seer.
Portrayed by Kim Myeong-su in the 2014 KBS TV series Jeong Do-jeon
- Queen Indeok of the Borjigin clan (? – 8 March 1365) (인덕왕후)
- Queen Sunjeong of the Goksan Han clan (? – 1376) (순정왕후 한씨)
- Royal Consort Hye-Bi of the Gyeongju Lee clan (? – 3 February 1408) (혜비 이씨)
- Royal Consort Ik-Bi of the Kaesong Wang clan (익비 왕씨)
- Royal Consort Shin-Bi of the Paju Yeom clan (신비 염씨)
- Royal Consort Jeong-Bi of the Jukju Ahn clan (? – 1428) (정비 안씨)
- Royal Concubine Ban-Ya (시비 반야)
- King U of Goryeo (25 July 1365 – 31 December 1389) (고려 우왕)