(written from a Production point of view)
John "Jack" Daniel Jefferies, Sr. (8 February 1936 – 25 March 2010; age 74), often credited simply as "John Jefferies", and nicknamed "Jack" by his family and co-workers, was a set designer on Star Trek: The Original Series and younger brother to the series' Art Director Matt Jefferies, the youngest of in total four brothers of which only the second oldest brother, Richard, did not work in Hollywood.
John Jefferies shared credit with Matt, who, in 1964, had brought in his "kid brother" (also calling him his "chief draftsman") to help him out with the design work for the first Star Trek pilot episode "The Cage", for designing the original phaser pistol and for which John had drawn up the construction blueprints. (Star Trek: The Magazine Volume 1, Issue 11, p. 22 ) John was able to do so, when he had some down-time at his hitherto employer, Columbia Pictures, though he later recalled keeping a low profile, wary of being accused of "nepotism" – rightfully so, as it was well known that Desilu Studios owner Lucille Ball could not abide with nepotism. John Jefferies also worked on the final construction of the USS Enterprise bridge, though he had to do this at nights, as he was working again on Columbia productions by day. 
John Jefferies returned three years later as set designer when the second season of the regular series went into production, after brother Matt and Gene Roddenberry convinced him to come aboard as a permanent production staffer, executing the designs his brother envisioned. He later recalled how he and his fellow set constructors pilfered the abandoned lot of RKO Pictures, by that time defunct and adjacent to that of Desilu Studios, for any and all items that could be used for the sets, and was therefore indispensable to his older brother, who had to adhere to ever tightening budgets, for keeping the set production costs as low as possible.
During his tenure on the series, a particularly close co-worker, especially in regard to set construction, was Set Decorator John Dwyer. (Inside Star Trek, issue 4, p. 3) To exemplify the down-to-earth approach the team had to observe, Jefferies elaborated, "Subconsciously, you were always on the search for something that you could use, and the trash bin was a goldmine on many occasions.(...)One of my favorites has always been the IBM Selectric typewriter. That was a new machine and Paramount had just bought a truckload of these things. And Office Service was busy installing these things in all the offices, and their packing was foam, foam shapes that were new. This was a new type of shipping material. And these they were throwing away. And we were roaming the streets one day and we saw this bin full of these Styrofoam shapes and grabbed them and I said, "Hey, we could make these work, you know. They're good shapes, they're light, we can cut them, change their shapes, put holes in them, put lights behind them, paint them, run pipes into them. You might see them in the hallway, you might see them in a room that was not established as a regular part of either the ship or a planet surface set." To this, Jonh Dwyer has added, "We stole moldings from the casting shop and put them in the engine room. And we painted soccer balls and used them at three different points on the engine room, sitting there, little domes that are painted gold, boom, boom, boom. And the big problem there was, keeping the actors from playing with them. Scotty used to come in and started to throw the thing around – he's a pretty good soccer player, was, you know – and I'm sitting there looking at him and Shatner booting the ball around, you know, "Guys, take it easy there. I got two more of them, but I don't wanna waste them, you're breaking the paint!"" (TOS Season 2 DVD special feature, "Designing the Final Frontier")
When complimented decades later for the believability of the sets he helped to build, Jefferies dryly responded, "Oh, you are kind if you believe it was believable, because I was not sure it was." It was he and his team who came up with the signage "GNDN" for the several color-coded pipings, seen throughout the interior of the Enterprise, which was the acronym for "Goes Nowhere, Does Nothing", renowned in Star Trek lore. (Star Trek: 45 Years of Designing the Future)
Upon termination of The Original Series, John Jefferies obtained ownership of several production assets, including one of the small USS Enterprise filming models, as well as one of the two Tholian ship studio models. However, he, along with his brother Matt, sold off virtually all of their Original Series production items, including his models, still in their possession in the Profiles in History The Star Trek Auction of 12 December 2001, in order to raise funds for the charitable organization "Motion Picture & Television Fund".
Shortly before his death, John Jefferies, together with fellow designers Joseph R. Jennings (who actually already knew and worked with John Jefferies on the Original Series), Herman F. Zimmerman and Scott Chambliss, were honored for their Star Trek contributions in a media event called the "Star Trek Designers Talk Trek History At Art Directors Guild Event", held on 27 September 2009 at the Egyptian Theater in Hollywood, and in which all designers discussed indepth their work on the franchise. The event was moderated by another Star Trek alumnus, Daren Dochterman.  For John Jefferies, this has been a particularly gratifying experience, as he has always been somewhat overshadowed by his more renowned brother Matt as far as his Star Trek contributions were concerned, though the latter had always tried to give his younger sibling credit where credits were due.  The tribute has also been a poignantly timely one, as he was already battling the effects of cancer, passing away only a half year later. Prior to his appearance there, he was interviewed five years earlier for one of the special features on the TOS Season 2 DVD set, and which must count as one of the earliest better published acknowledgments of his Star Trek contributions.
Career outside Star Trek
Jefferies was born in Richmond, Virginia and grew up in New Jersey. Like his older brother Matt, John was a lifelong aviation enthusiast, having owned several airplanes throughout his life and has, again like his brother, also served in the United States Air Force. Directly after his release from the service Jefferies moved to Los Angeles, employed as design engineer at Lockheed's Missiles and Space Division, where he worked on the Discovery, Mercury, and Apollo projects.
He began his Hollywood career in 1962, when his older brother Philip, persuaded him to join his two older brothers, already working there, in the motion picture industry at a time when art directors/production illustrators were in relatively short supply. John was the last of the Jefferies brothers, Philip persuaded to make the move to Hollywood, the older one, Matt, already brought over by him in 1957. (Beyond the Clouds, pp. 211, 213, 222) Among his first recorded movie projects were the 1966 film The Chase, whose cast included Paul Williams, and the 1967 classic Guess Who's Coming to Dinner. Both John and Matt Jefferies worked on the 1967 Walt Disney film The Happiest Millionaire. While employed by the Star Trek franchise, Jefferies continued to work on movie projects, when the series was in hiatus, such as the musicals Funny Girl (1968, on which Dick Rubin was property master) and Hello, Dolly! (1969, with set dressings by Craig Binkley). 
John later worked as a set designer on the 1970 film Catch-22 while Philip Jefferies was the film's art director. During production, Philip became ill and was replaced by Matt. John also took over some of Philip's responsibilities on the film.  On Catch-22, all three brothers were working under production designer Harold Michelson, who later headed the art department on Star Trek: The Motion Picture. From the 1970s until 2001, Jefferies was an art director and production designer on such television shows as I Dream of Jeannie (as an uncredited set designer), Baa Baa Black Sheep (on which John Larroquette and James Whitmore, Jr. were regulars), The Greatest American Hero, Hardcastle and McCormick (starring Brian Keith and Daniel Hugh Kelly), Matlock, and JAG, the latter of which his last recorded motion picture industry credit. In his later career, he was also a set designer on such films as BASEketball, I Still Know What You Did Last Summer, and Austin Powers: The Spy Who Shagged Me. He was also the art director on the 2001 film Just Visiting, which featured Malcolm McDowell.
Jefferies' daughter married the son of film editor Bruce Schoengarth.
John Jefferies at one time served as president of the International Alliance of Theatrical Stage Employes (IATSE) Set Designers and Model Makers. Jefferies died on 25 March 2010 due to complications from lung cancer, the illness that took his older brother Phil (31 May 1925 – 6 April 1987; age 61) years earlier. Very much in the spirit of his 2001 auction, John Jefferies stipulated that in lieu of flowers, donations be made in his name to the American Cancer Society or to another favorite charity. He was 74 years old and was survived by his wife Dolores and six children. 
Brother Richard has written a biography on Matt, which in elaborate annotations also followed John's (and Philip's) career and life, though, obviously, not as detailed as that of his older brother.
Star Trek interviews
- TOS Season 2 DVD – special feature, "Designing the Final Frontier", interviewed on 1 June 2004
- Star Trek: 45 Years of Designing the Future, 2009