David McCallum, the beloved actor who played Dr. Donald "Ducky" Mallard on "NCIS," has passed away at the age of 90. Before "NCIS," McCallum made his first appearance as Ducky in the "JAG" backdoor pilot episode. He went on to portray Ducky for a remarkable 457 episodes on "NCIS." In addition, McCallum made guest appearances on "NCIS: New Orleans" in two episodes. His dedication and talent made him a cornerstone of the "NCIS" franchise.