born and educated in new york city under private tutors, gill early showed interest in natural history. he was associated with j. carson brevoort in the arrangement of the latter's entomological and ichthyological collections before going to washington d.c. in 1863 to work at the smithsonian institution. he catalogued mammals, fishes and mollusks most particularly although maintaining proficiency in other orders of animals. he was librarian at the smithsonian and also senior assistant to the library of congress.