Lenard is best known for his appearances in the Star Trek franchise, particularly in the role of Sarek, the father of Spock (Leonard Nimoy). However, his first Star Trek appearance was in the first season of Star Trek: The Original Series, playing the first Romulan ever seen in the series, in the episode "Balance of Terror" (1966). He originated the character of Sarek in the second-season episode "Journey to Babel" (1967), and provided the voice of Sarek in the Star Trek: The Animated Series episode "Yesteryear" (1973). He later played an ill-fated Klingon Captain in Star Trek: The Motion Picture (1979), which gave him the distinction of being the first actor to play a Romulan, a Vulcan, and a Klingon in Star Trek. He later reprised the role of Sarek in three of the Star Trek feature films: Star Trek III: The Search for Spock (1984), Star Trek IV: The Voyage Home (1986), and Star Trek VI: The Undiscovered Country (1991), and provided the voice of young Sarek in a brief flashback sequence in Star Trek V: The Final Frontier (1989). Additionally, he appeared as elderly Sarek in the third-season Star Trek: The Next Generation episode "Sarek" (1990) and the fifth-season episode "Unification: Part 1" (1991).