Yu first served as a gapsa (갑사; 甲士) or armoured soldier, who guarded Geonchunmun Gate.
When Yi Si-ae's Rebellion broke out, he volunteered to serve the government forces in suppressing the rebels.
He was quickly promoted to the position of byeongjo jeongnang (병조정랑; 兵曹正郞) or a section chief in the Ministry of Defence by King Sejo.
[1] When Sejo's son Yejong ascended to the throne, Yu Jagwang accused Nam I of plotting treason and was rewarded as a first class merit subject.
In 1498, Yu played a key role in the First literati purge during the reign of Yeonsangun, accusing Kim Il-son of writing history drafts that were negative on King Sejo.