Updated: September 28, 2026

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.

"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. 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 carte blanche 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. Dr. Smith's best known tropes included spitefully calling The Robot epithets such as "bubble-headed booby" and "clamoring clod".