Born in 1357 as the second son of Yi Seong-gye (posthumously King Taejo) and his first wife Lady Han, he was a prudent, generous, brave, and able military officer.

During the latter days of the declining Goryeo dynasty, Jeongjong followed his father to various battlefronts and fought at his side.

Chief State Councillor Jeong Do-jeon also backed him as successor, causing much resentment in the other princes.

In 1398, Taejo's fifth son, Yi Bang-won (posthumously King Taejong) led a coup along with many military officers and killed his two younger half-brothers, Jeong Do-jeon, and many of his faction.

King Jeongjong, knowing that he was a mere political figurehead for his younger brother, appointed him as crown prince and abdicated months later.