Dr. Donald "Ducky" Mallard is the chief medical examiner at NCIS. Dr. Mallard is a Scottish-born doctor, who has been long-time friends with Gibbs. He has a "second talent", as Gibbs calls it, to be able to read people, which he expands in season four by studying psychology. In cases without actual bodies, he assists by using his psychological training to decipher the clues left by the perpetrators. His skill in reading behavior proves to be crucial when establishing modus operandi and motive.

Dr. Mallard is an eccentric character who often talks to the deceased ("their bodies tell me a great deal; it helps to reciprocate") and rambles to the living with many long personal remembrances or historical accounts, but is a kind man at heart. Tony described him, "sometimes his head is connected directly to his mouth". He also calls co-workers by their full first names, with the exceptions of Abby, Gibbs, and medical assistant Jimmy Palmer, whom he addresses as Abigail (or Ms. Sciuto), Jethro, and Mr. Palmer, respectively (although he does refer to Palmer by his first name, Jimmy, when concerned for him, as revealed in "About Face" and "Detour"). He also refers Jenny Shepard and Leon Vance by their job title, "Director".