He was the son of Muyeol and the younger brother of Munmu, the twenty-ninth and thirtieth kings respectively of Silla.
In 653 King Muyeol entrusted his son In-mun with the diplomatic mission of securing a Tang military alliance against Silla's rival Baekje.
The 12th century Korean history Samguk Sagi contains a brief biography of Kim In-mun.
The Tang emperor sent a special delegation to conduct Kim's body back to Silla.
The 13th century Samguk Yusa relates that Kim In-mun died on board ship en route back to Silla, but considering the rather detailed account of Kim's death and subsequent funeral embassy found in the Tang histories and repeated in the Samguk Sagi, this account seems doubtful.