Born in 1417 as Yi Yu, King Sejong the Great's second son, he showed great ability at archery, horse riding and martial arts. He was also a brilliant military commander, though he never went to the battlefront himself. He became Grand Prince Suyang in 1428, the name by which he was better known. Following King Sejong's death, Suyang's ill brother, Munjong, took the throne but soon died. The crown passed to his 12-year-old son, Danjong. The new emperor was too young to rule the nation, and all political processes were controlled by then-premier Hwangbo In and General Kim Jongseo, who was vice-premier. As Kim Jongseo and his faction used the chance to extend the power of court officials against many royal family members, the tension between Kim and Suyang greatly increased; not only Suyang himself, but his younger brother, Grand Prince Anpyeong, also sought an opportunity to take control of the kingdom. Suyang surrounded himself with trusted allies, including his famous adviser, Han Myung-hoi. Han advised Suyang to take over the government in a coup, and on 10 November 1453, he killed Kim Jongseo and his faction, thereby taking the reins of power into his own hands. After the coup he arrested his own brother, Anpyeong, first sending him into exile, then sentencing him to death.
Reign
Finally in 1455 he forced his powerless young nephew, Danjong, to abdicate, declaring himself seventh king of the Joseon dynasty. Later he demoted Danjong to prince and ordered him to be poisoned after his younger brother, Grand Prince Geumsung, and later six scholars including Seong Sam-mun, Pak Paeng-nyeon, and Yi Gae plotted to remove Suyang from power in an attempt to put Danjong back on the throne. Despite having snatched the throne from his young nephew and killing many people in the process, he proved himself one of the most able rulers and administrators in Korean history. First, he strengthened the monarchy established by King Taejong by weakening the power of the prime minister and bringing staff directly under the king's control. He also strengthened the administrative system, which had also been introduced by Taejong, enabling the government to determine exact population numbers and to mobilize troops effectively. Just like Taejong, he was a hardliner with regards to foreign policy and attacked the Jurchens on the northern front in 1460 and 1467. He also revised the land ordinance to improve the national economy. He executed scholars from King Sejong's era for plotting against him, but encouraged publication of history, economics, agricultural, and religious books.
Last years
Most importantly, he compiled the Grand Code for State Administration, which became the cornerstone of dynastic administration and provided the first form of constitutional law in a written form in Korea. He died in 1468, and the throne passed to his sickly son, Yejong.
Sejo of Joseon compiled a number of books based on his interests. One includes Seokbosangjeol, a biography of Gautama Buddha. He also created two other books:
