Nevil Jermyn was the second son of the famous anthropologist and explorer Robert Jermyn, and the grandfather of Arthur Jermyn. He had two siblings, one younger and one older, who are unnamed and were never publically seen due to physical and mental deformities. Nevil himself was described as a "singularly repellent" person who ran away with a common dancer in 1849, but was pardoned by his father upon his return in the following year. He returned as a widower with an infant son named Alfred, the future father of Arthur. (HPL: "Facts Concerning the Late Arthur Jermyn and His Family")
When an explorer named Samuel Seaton visited the Jermyn home in 1852 with a manuscript of notes collected in Africa, Robert discovered to his horror that there was a simian strain in his own family originating from a union between his grandfather Wade Jermyn and a female member of a race of African white apes. The unhinged Robert murdered Seaton along with his own three children including Nevil, but the two year old Alfred survived. (HPL: "Facts Concerning the Late Arthur Jermyn and His Family")