Jimmy Palmer first appears in the episode "Split Decision". After Gerald Jackson was incapacitated, Palmer became Mallard's medical assistant both in the field and in the morgue. In the episode "About Face", Jimmy became a central character of the episode who must recover his memory to find a suspect to the murder case and his attempted killer. He self-identifies as a sufferer of a "mild" case of diabetes mellitus in the episode "In The Dark". He often seems intimidated by Gibbs, especially when he goes into an explanation of something that was not requested. Part of the reason that Dr. Mallard and he are often not at the crime scene until well after Gibbs and his team arrive is related to Dr. Mallard's emphasis on Jimmy being a horrible driver and always getting lost, although Jimmy tries to defend himself by pointing out that Ducky is the one with the map. He was named after former Baltimore Orioles pitcher Jim Palmer, but does not like baseball.

Like Ducky, Palmer speaks to the dead though he is far more irreverent, sometimes raising eyebrows in the process.