Niu participated in the Battle of Jiangling, during which he volunteered to lead 300 cavalry to attack the enemy, but was only saved by Cao Ren when he got surrounded.
Years later, he participated in the counter-attack against the invading Shu Han forces in 231 and scored a victory over Zhuge Liang's unit, and pursued him to Mount Qi. In 234, he soundly defeated the Shu Han general Ma Dai on the battlefield, and chopped off thousands of enemies' heads. In 238, he followed Sima Yi in his campaign against Gongsun Yuan, and was promoted to General of the Rear.
However, his accomplishments during his later years had only earned him the apprehension from the Simas, thus he was ordered to commit suicide by drinking poison. The exact reasons for his death as well as the year he died remain disputed. The Yuan Shi Juan states that the incident happened "after the Year of the Horse" and that Niu Jin was poisoned by drinking toxic wine. Other records state that it was because Niu Jin committed adultery with one of the Sima relatives and was assassinated with poison arrows "during the Year of the Cow". This account is somewhat supported by Wei's records as they state that an illegitimate child was born in the Sima family. A story from unknown origin also states that Niu Jin was killed since he was actually a distant relative of the former emperor; his death date and the accuracy of this statement are difficult to verify.