Commander Ed Straker (Ed Bishop) is a former American Air Force Colonel, pilot and astronaut originally from Boston, Massachusetts, who organised S.H.A.D.O. following a series of UFO attacks in 1970. Straker masquerades as the head of Harlington-Straker Film Studios, S.H.A.D.O. Headquarters being located directly below the studio.
Colonel Alec Freeman (George Sewell) a former pilot and intelligence officer, is S.H.A.D.O.'s first officer (and very first operative recruited into S.H.A.D.O. by Commander Straker) for 17 episodes in the series (George Sewell left UFO following the change of studios, being later unavailable when series production resumed at Pinewood Studios). Colonel Freeman is Commander Straker's closest friend and right-hand man and, occasionally, his muscle.
Colonel Paul Foster (Michael Billington) is introduced in the second episode, "Exposed". A former test pilot, his plane was critically damaged when S.H.A.D.O.'s Sky One intercepted and destroyed a UFO in close proximity to Foster's jet. His persistent investigation of the incident threatened to expose S.H.A.D.O.'s existence so Straker offers him a position within S.H.A.D.O.
Lieutenant Gay Ellis (Gabrielle Drake), seen as Moonbase commander during the first half of the series. Lt. Ellis is occasionally portrayed as lacking self-confidence, and at other times as a take-charge officer. She is briefly reassigned to S.H.A.D.O. HQ when it is suggested that she may be romantically involved with interceptor pilot Mark Bradley ("Computer Affair"). She also appears to be attracted to Ed Straker, though nothing comes of this.
General James L. Henderson (Grant Taylor), Straker's superior officer, serves as the president of the International Astrophysical Commission, which is a front for S.H.A.D.O. and is responsible for obtaining funds and equipment from various governments to keep S.H.A.D.O. operational. Straker and Henderson clash frequently over the needs of S.H.A.D.O. and economic realities.
Colonel Virginia Lake (Wanda Ventham) first appears in the opening episode of the series ("Identified"), as a S.H.A.D.O. scientist. A computer specialist, she was a member of the Westbrook Electronics "Utronics" tracking device design team. During the last eight episodes, Colonel Lake returned to take over the post of S.H.A.D.O. first officer, replacing Colonel Freeman.
Captain Peter Carlin (Peter Gordeno), during the first third of the series, Carlin is the commander of the submarine Skydiver and pilot of its interceptor aircraft, Sky One. In 1970, Carlin and his sister found a UFO and were attacked; he was shot and wounded and his sister vanished. He joined S.H.A.D.O. in the hope of finding out what happened to his sister, and eventually learned that her organs had been harvested in the pilot episode "Identified".
Lieutenant Nina Barry (Dolores Mantez) is one of Straker's first recruits into S.H.A.D.O. Lieutenant Barry works as a space tracker at Moonbase and later replaces Lieutenant Ellis as its commanding officer. She also serves aboard Skydiver at one point ("Sub-Smash").
Captain Lew Waterman (Gary Myers) was initially an interceptor pilot on the Moon but is later promoted to captain and replaces Captain Peter Carlin as commanding officer of Skydiver and pilot of Sky One.
Lieutenant Keith Ford (Keith Alexander) is a former television interviewer who became a founding member of S.H.A.D.O. and its main communications officer. Actor Keith Alexander left the series after the production break, so the character disappears at the two-thirds mark of the series.
Lieutenant Ayshea Johnson (Ayshea Brough) a S.H.A.D.O. headquarters officer in 14 episodes and later becomes S.H.A.D.O.'s communications officer following the departure of Lieutenant Ford.
Doctor Douglas Jackson (Vladek Sheybal) is the S.H.A.D.O. psychiatrist and science officer. He serves a number of capacities within S.H.A.D.O., including acting as prosecution officer during the court martial of Paul Foster. It is implied that "Douglas Jackson" is not the character's birth name, as he speaks with a strong Eastern European accent.
Lieutenant Joan Harrington (Antonia Ellis) another Moonbase operative, was one of the organisation's earliest recruits, as seen in "Confetti Check A-O.K.".
Miss Ealand (Norma Ronald) is a S.H.A.D.O. operative who masquerades as Straker's movie studio secretary. She is the first line of defence against anyone entering S.H.A.D.O. Headquarters via Straker's office/elevator. The character is not seen in most of the post-studio change episodes, being replaced in two episodes by a Miss Holland, played by Lois Maxwell.
Lieutenant Mark Bradley (Harry Baird) is a Caribbean-born interceptor pilot based on the Moon. He becomes romantically involved with Lt. Ellis for a time, leading to a temporary assignment at S.H.A.D.O. H.Q. on Earth, and later briefly assumes the position of Moonbase commander. Baird left the series after filming four episodes, but appeared in stock footage in two later episodes.