As the eldest son, he was confirmed as successor to King Asin, in 394. His queen was Lady Palsu of the Hae clan.

Jeonji spent much of his youth in the Wa kingdom of Yamato Japan as hostage, going there in 397. Upon his father』s death, he returned home to find that his uncle Seollye had murdered Hunhae, Asin's other brother, and usurped the throne. Hae Chung, an inhabitant of Hanseong, warned him not to enter the capital. Shortly thereafter, Seollye was killed and Jeonji made king. Presumably out of gratitude for this, Jeonji made several members of the Hae clan ministers, as well as marrying Lady Palso of the Hae clan. This put an end to the royal family』s close ties to the Jin clan.

The traditional dates of Jeonji's rule are based on the Samguk Sagi. On the basis of more contemporaneous Chinese records, Best (1979) has suggested that the years 405–414 are more plausible.

According to the Samguk Sagi, in 406 Baekje sent a tribute mission to the Chinese court of Eastern Jin. It is the first mention of such a mission in more than twenty years, and may indicate that the country had become more secure against Goguryeo. It would have been typical to send such an embassy to inform the Chinese court that a new king had taken power. However, this visit is not confirmed by Chinese sources. In 416 Jin sent envoys to grant the title "General Stabilizing the East" to Jeonji(Yeo Yeong).

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394年2月立为阿莘王太子。397年作为人质前往倭国。405年9月阿莘王死，由次子训解监国，欲遣使往倭国迎回腆支王。但不久训解为腆支王的末弟碟礼杀害，碟礼自立为王。腆支王在倭国军队护卫下回到百济，诛杀碟礼即位。

407年以庶弟馀信、外戚解须、解丘三人为佐平（1等官），并执朝政。次年设置上佐平一职，并由馀信出任此职。416年受东晋王朝「使持节、都督、百济诸军事、镇东将军、百济王」的封号。417年7月在百济东北边境修建沙口城，调整与高句丽对抗的态势。