In an apparent homage to his earlier role, Harris played a similarly pompous diplomat on "Get Smart" in 1970. His female assistant was named Zachary. Harris also guest-starred on "The Ghost & Mrs. Muir". Harris was cast over two other actors for the role of Dr. Zachary Smith, the evil and conniving enemy agent on "Lost in Space". The character did not appear in the original 1965 pilot episode for CBS, nor did The Robot. The series was already in production when Harris joined the cast, and starring/co-starring billing had already been contractually assigned. Harris successfully negotiated to receive "Special Guest Star" billing on every episode. Bill Mumy said about Harris' role in his first episode, "It was actually implied that this villainous character that sabotaged the mission and ended up with us was going to be killed off after a while." Mumy added, "Jonathan played him as written, which was this really dark, straight-ahead villain." The series was successful upon its debut, and midway through the first season, Harris began to rewrite his own dialogue to add more comedy, because he felt that his strength was in portraying a comic villain. Due to Harris's popularity on the show, Irwin Allen approved his changes and gave him as a writer. Harris subsequently stole the show, mainly via a seemingly never-ending series of alliterative insults directed toward The Robot, which soon worked their way into popular culture. Smith's best-known tropes included spitefully calling The Robot epithets such as "bubble-headed booby" and "clamoring clod". According to Bill Mumy, Harris moved quickly to develop the character: "And we'd start working on a scene together, and he'd have a line, and then in the script I'd have my reply, and he'd say, 'No, no, no, dear boy. No, no, no. Before you say that, The Robot will say this, this, this, this, this, this, and this, and then, you'll deliver your line.'" Mumy also said of Harris' portrayal, "He truly, truly singlehandedly created the character of Dr. Zachary Smith that we know — this man we love to hate, coward who would cower behind the little boy, 'Oh, the pain! Save me, William!' That's all him!" A strong bond developed between Harris, Mumy, and some of the rest of the cast during the series' three-year tenure.