Updated: March 17, 2022

Dr. John Robinson: (Guy Williams) The expedition commander, a pilot, and the father of the Robinson children. He is an astrophysicist who also specializes in applied planetary geology.

Dr. Maureen Robinson: (June Lockhart) John's biochemist wife. Her role in the series is often to prepare meals, tend the garden and help with light construction, while adding a voice of compassion. Her status as a doctor is mentioned only in the first episode.

Major Don West: (Mark Goddard) The military pilot of the Jupiter-2, he is Dr. Smith's handsome long suffering space partner, intemperate and intolerant adversary. His mutual romantic interest with Judy was not developed beyond the first few episodes. In the un-aired pilot, "Doctor Donald West" was a graduate student astrophysicist and expert in interplanetary geology, rather than a military man.

Judy Robinson: (Marta Kristen) The oldest child, about 19 years old at the outset of the series. She planned a career in musical theater but went with her family instead.

Penny Robinson: (Angela Cartwright) An 11 year old, she loves animals and classical music. She acquires a chimpanzee-like alien pet that made one sound, "Bloop". While it is sometimes remembered by that name, Penny had named the creature Debbie. Most of Penny's adventures have a fairy-tale quality, underscoring her innocence.

Will Robinson: (Billy Mumy) A 9-year-old child prodigy in electronics. Often, he is a friend to Smith when no one else is. Will is also the member of the family closest to the Robot.

Dr. Zachary Smith: (Jonathan Harris) A Doctor of intergalactic environmental psychology, expert in Cybernetics and an enemy agent, roles that are rarely mentioned after the initial episodes. In the pilot episode, he is shown in uniform with colonel's eagles but is invariably addressed by his academic, vice military, rank. His attempt to sabotage the mission strands him aboard the Jupiter 2 and results in its becoming lost. By the end of the first season the character becomes permanently established as a foolish, self-serving, impulsive, scheming coward but not at the degree displayed in the latter two seasons. His maudlin ways and clever dialogue add a unique dimension. His best lines are in response to the "straight man" Robot. Despite having a Special Guest Star appearance for every episode, Smith is the pivotal character of the series.

The Robot: The Robot is a Class YM-3 Model B9, General Utility Non-Theorizing Environmental Control Robot, which had no given name. Although a machine endowed with superhuman strength and futuristic weaponry, he often displayed human characteristics such as laughter, sadness, and mockery as well as singing and playing the guitar. The Robot was performed by Bob May in a prop costume built by Bob Stewart. The voice was dubbed by Dick Tufeld, who was also the series' narrator. The Robot was designed by Robert Kinoshita, whose other cybernetic claim to fame is as the designer of Forbidden Planet's Robby the Robot. Robby appears in LIS #20 "War of the Robots", and the first episode of season three; "Condemned of Space".