Before his death, King Seonjo named Prince Gwanghae as his successor. However, Lyu Young-gyong of the Lesser Northerners faction concealed the royal succession document and planned to install Grand Prince Yeongchang as king. This plot was discovered by Jeong In-hong of the Great Northerners faction, leading to Lyu's execution and Yeongchang's arrest and subsequent execution.

As king, Gwanghae sought to unify various political factions in his court, but faced opposition from the Greater Northerners, including Yi I-cheom and Jeong In-hong. This faction systematically removed members of other factions, particularly the Lesser Northerners. In 1613, they targeted Grand Prince Yeongchang and his grandfather Kim Je-nam, both of whom were executed. Queen Inmok, Yeongchang's mother, was stripped of her title and imprisoned in 1618. Gwanghae, despite being the official head of the government, was powerless to intervene.

Gwanghae was a talented and pragmatic ruler, focusing on rebuilding the country. He sponsored the restoration of documents, revised land ordinances, redistributed land to the people, and ordered the rebuilding of Changdeok Palace and other palaces. He also reintroduced the hopae identification system. In foreign policy, Gwanghae sought to balance relations between the Ming Empire and the Manchus, sending troops to aid the Ming against the Manchus but negotiating peace with the Manchus after their victory. He reopened trade with Japan in 1609 and restored diplomatic relations in 1617.

Domestically, Gwanghaegun implemented the Daedong law for easier tax payment in Gyeonggi Province, encouraged publishing, and oversaw the writing of important works like the medical book Dongui Bogam. Tobacco was introduced to Korea during his reign and became popular among the aristocracy.

Gwanghaegun's reign ended with his dethronement by the Westerners faction in a coup led by Kim Yu on April 11, 1623. He was initially confined on Ganghwa Island and later on Jeju Island, where he died in 1641. Unlike other Joseon rulers, he does not have a royal mausoleum, and his remains are buried in a humble site in Namyangju, Gyeonggi Province. His successor, King Injo, implemented pro-Ming and anti-Manchu policies, leading to two Manchu invasions.