(written from a Production point of view)
Ralph Abbott Senensky (born 1 May 1923; age 98) built a long career directing television series and the occasional telefilm beginning in 1958. During that career he directed seven episodes of Star Trek: The Original Series, spanning the time period 1967-1968, and encompassing episodes from all three seasons.
Senensky was originally scheduled to direct "The Devil in the Dark", but the production schedule was switched with "This Side of Paradise".  The production staff was well-satisfied with his work on the episode, and he was hired to direct other episodes of the second and third seasons. (Inside Star Trek: The Real Story, p 208) During the second season, Senensky was hired as the third "rotating director", filling in when neither Joseph Pevney nor Marc Daniels were available. (These Are the Voyages: TOS Season Two)
However, Senensky was fired by producer Fred Freiberger midway through shooting "The Tholian Web" because he went over schedule, and although almost all of the footage he shot remained in the episode, he was not credited. (The Star Trek Interview Book)  Brief footage shot by Senensky for "This Side of Paradise" also appeared in TNG: "Relics".
Senensky has been featured as recently as 2016, when he was interviewed for the Inside the Roddenberry Vault (all three parts) special feature documentary for the Star Trek: The Original Series - The Roddenberry Vault Blu-ray Disc release, where delved into detail on his involvement with the episodes "This Side of Paradise","Metamorphosis" and "Return to Tomorrow", three of the twelve episodes prominently featured on the release. Co-interviewer Denise Okuda has expressed astonishment on the sharpness and vividness of Senesky's recollections, having stated, "And he was amazing! He’s 92 years old and his memory is sharper than mine. He could tell you specific days – "oh yeah, we shot that on a Tuesday," and "the sun was going down, and it was mostly on a planet..." 
Senensky himself has divulged a plethora of minutiae on his involvement with The Original Series on his own specialty blog "Ralph's Cinema Trek: A Journey in Film", which he put online in 2010, and where he has the second season episode "Metamorphosis" stated as his personal favorite, citing its writer Gene Coon as "incomparable".
Career outside Star Trek
Graduating from the Pasadena Playhouse, he began his career directing in the theater, then worked as a production supervisor on Playhouse 90 and as an assistant producer on Dr. Kildare. His work stretches from The Twilight Zone through many popular sitcoms and dramas of the 1960s and '70s.
In addition to Star Trek: The Original Series, he worked on dramas such as Dr. Kildare, Route 66 (including "In the Closing of a Trunk", featuring Jon Lormer and Guy Raymond), Naked City (including "Alive and Still a Second Lieutenant", written by Shimon Wincelberg and "Color Schemes Like Never Before", featuring Lou Antonio; the series also featured Lawrence Dobkin as narrator and Andrew Laszlo as co-cinematographer), Arrest and Trial (starring Roger Perry), The Fugitive (including "When the Bough Breaks", featuring Jud Taylor, "An Apple a Day", featuring Kim Darby, and "When the Wind Blows", featuring Harry Townes), The F.B.I. (starring Stephen Brooks, including "The Plunderers", featuring Bill Erwin, "The Escape", featuring Steve Ihnat and Paul Comi, "The Assassin", featuring William Windom, Ted Knight, and James B. Sikking, "Anatomy of a Prison Break", featuring George Murdock, Vic Perrin, and Paul Winfield, "The Death Wind", featuring Peter Mark Richman, Bill Quinn, and Garrison True, "The Raid", featuring Seymour Cassel, Ken Lynch, Dallas Mitchell, and Rudy Solari, "The Courier", featuring Keye Luke and Gene Lyons, "Game of Terror", featuring James B. Sikking, "End of a Hero", featuring Lee Meriwether, "A Second Life", featuring George Sawaya, and "Arrangement with Terror", featuring Roger Perry), Kraft Suspense Theatre (including "The Easter Breach", starring Richard Beymer), 12 O'Clock High (starring Frank Overton and Robert Lansing, including "To Heine – With Love", featuring Stewart Moss and "The Trap", featuring David Frankham), The Wild, Wild West (produced by Gene L. Coon), Mannix, Ironside, I, Spy ("This Guy Smith", featuring Diana Muldaur), Insight with Booker Bradshaw and Jeffrey Hunter, Mission: Impossible ("The Train", featuring William Windom and William Schallert), Night Gallery (including "The Ghost of Sorworth Place", featuring Jill Ireland and Richard Kiley), Planet of the Apes ("The Tyrant", featuring Mark Lenard, Percy Rodrigues, Joseph Ruskin, Gary Combs, Tom Troupe, and cinematography by Jerry Finnerman), The Waltons (including "The Chicken Thief", featuring John Crawford and Meg Wyllie, "The Conflict", featuring Bill Irwin, Bill Quinn, Morgan Woodward, and Paul Fix, and "The Warrior", also with John Crawford), Barnaby Jones (starring Lee Meriwether and theme music by Jerry Goldsmith, including "Murder Once Removed", featuring Bill Quinn, and "To Denise, with Love and Murder", featuring Vince Howard), Casablanca (starring David Soul), and Paper Dolls (starring Richard Beymer, Jonathan Frakes, Terry Farrell, and Geoffrey Blake), and comedies such as The Courtship of Eddie's Father (produced by and co-starring James Komack, including "Gentleman Friend" with George Takei and "The Promise" with Meg Wyllie), The Partridge Family, and Eight is Enough as well as many other television series.
His 1974 TV movie The Family Kovack (written by Adrian Spies) starred James Sloyan and Andrew Robinson and featured Peter Brocco. Seven years later he worked with Andrew Robinson again in the failed television pilot Big Bend Country, which also featured Anne Haney. In 1970 Senensky co-directed Double Jeopardy, the TV movie pilot for the series Dan August, featuring Susan Oliver, Jerry Ayres, Ned Romero, and Fritz Weaver. His only feature film credit, Harper Valley P.T.A. (1978, finished by Richard C. Bennett after Senensky was fired from the project) starred Ronny Cox and featured Clint Howard. Senensky also directed the pilot episode of Dynasty, "Oil", which featured Peter Mark Richman, Lee Bergere, Jerry Ayres, Ellen Geer, Julie Parrish, and Paul Sorensen.
In the mid-1980s, he briefly returned to the theater, where he directed the plays You Can't Take it With You and Watch on the Rhine. Currently, Senensky resides in Carmel-by-the-Sea, California, enjoying retirement and writing his blog.
In 2013, Senensky directed his first film project in more than two decades, an independent production titled The Right Regrets. 
Senensky is currently the oldest living production staff member who worked on Star Trek.
Star Trek credits