Leigh Lawson - he appeared in the season 2 episode “One Moment of Humanity”. He has appeared on stage, film and TV though mostly in the U.K. He’s husband of model actress Twiggy.
Christopher Lee - in his long career he has starred in many Hammer horror films, most notably Dracula. With 281 acting credits on IMDB, his career include famous roles in both Star Wars and Lord of the Rings.
Roy Marsden - About 10 years after appearing in Space 1999, Marsden began his longer tenture of playing P. D. James’s detective Adam Dalgliesh (for 15 years).
Leo McKern - I best remember McKern for his recurring role on “The Prisoner” TV series where he played the mysteries Number 2. His most famous role must be in “Rumpole of the Bailey” which he played for 15 years.
Ian McShane - McShane has had a long career in film & TV. For a while he was best know for the TV show “Lovejoy” but he since went on to make the memorable “Deadwood”. And he’ll soon be seen in “American Gods”. Recently he has appeared in the “John Wick” film franchise with Keanu Reeves.
David Prowse - a few years after Prowse did some stunt work on “Space 1999” he was one of two actors who portrayed Darth Vader in “Star Wars”.
David Robb - I recognized the name and the face looked vaguely familiar. When I checked I realized David Robb played the Doctor on “Downton Abbey”.
Kathryn Leigh Scott - Scott appeared in the classic TV series “Dark Shadows”. After the show ended appeared in an episode on “Space 1999” and later on “Star Trek Next Generation” as a proto-Vulcan.
Carolyn Seymour - I best remember Seymour for the terrific post apocalyptic British show “Suvivors”. In the sci-fi vein she had a recurring role on STNG as a Romulans.
Catherine Schell - Before joining the cast as a regular in season 2, Schell made a guest appearance in the “Guardian of Piri”.
Patrick Troughton - None other than the Second Doctor (Who) appeared as a dying alien ruler on “Space 1999”.
Art Wallace wrote for Space 1999 but his genre career includes writing for “Dark Shadows”, “Star Trek” TOS, and the “Planet of the Apes” TV show.