He was the second son of King Seonjo. When Japan invaded Korea to attack the Ming Empire, he was installed as Crown Prince. While the king fled north to the border with Ming, he set the branch court and fought defensive battles.
Although it brought prestige to him, his position was still unstable. He had the elder but incompetent brother Imhaegun (臨海君) and a younger but legitimate borther Yeongchangdaegun (永昌大君), who was supported by the Lesser Northerners faction (小北). Fortunately for him, King Seonjo's abrupt death made it impossible for his most favorite son Yeongchang-taegun to succeed the throne.
As soon as he ascended to the throne with the support of the Great Northerners faction (大北), he arrested Imhaegun, who died in the next year. Although his succession was approved by the Ming Emperor several years late, he imprisoned Yeongchang-taegun and his mother, the queen dowager in 1613. At the same time he suppressed the Lesser Northerners.
Despite the bad reputation in later times, he was a talented and realistic politician. He endeavored to restore the country and sponsored restoration of documents.
In foreign affairs he sought a balance between the Ming Empire and the Manchus. Since he realized Joseon was unable to compete with the Manchu military power, he tried to keep friendly relationship with the Manchus, while the kingdom was still under the suzerainty of Ming, which angered Ming and dogmatic Confucist Koreans. However the critically worsened Manchu-Ming relatinship forced him to send ten thousand soldier to aid Ming in 1619. However, the Battle of Sarhū ended in Manchu's overwhelming victory. The Korean General Gang Hong-rip lost two-thirds of his troops and surrendered to Nurhaci. Gwanghaegun negotiated independently for peace with the Manchus and managed to avoid another war.
In 1623 Gwanghaegun was deposed by the Westerners faction (西人) with a coup. He was confined first on Ganghwa Island (江華島) and then Jeju Island (濟州島), where he died in 1641. He does not have a mausoleum like the other Joseon rulers. His burial place is unknown. The Westerners faction installed Injo as the 16th king and took pro-Ming and anti-Manchu policies, which brought two Manchu invasions.