During Kirk's second year of command, in 2266, Hadley worked as a command division assistant to Chief Engineer Lieutenant Commander Montgomery Scott. On stardate 1512.2 of that year, he was running in a corridor to report to Scott when the Enterprise went to red alert and engaged the First Federation, and briefly assisted Scott on the bridge at the life support station. Hadley was one of the witnesses to then-navigator Lieutenant Dave Bailey's nervous breakdown on stardate 1514.0, and on stardate 1514.1, Kirk's use of the imaginary substance corbomite to bluff Balok to back down with a threat of exploding and destroying the First Federation ship, the Enterprise, and all aboard the Enterprise. (TOS: "The Corbomite Maneuver")
The first time Hadley was stationed as a navigator was from stardate 2821.5 to 2823.1 when the shuttlecraft Galileo was lost in the Murasaki 312 quasar. This was also one of the few times when Hadley was seen laughing, when Commander Spock wouldn't admit that the flare he used to draw attention to the Galileo was a purely emotional response. Hadley laughed with the rest of the bridge crew at Spock's non-committal of an answer. (TOS: "The Galileo Seven")
Hadley first worked as a full-time bridge officer in 2267, when he started at the life support system station. This was also the first time he temporarily manned the helm. On stardate 2125.7, after Kirk and Lieutenant Hikaru Sulu vanished from the bridge while they were briefly investigating a rogue planet composed of iron-silica, Hadley walked down the steps to the helm station and took control of the helm after Lieutenant Carl Jaeger, Dr. Leonard McCoy, and Lieutenant DeSalle beamed down to the planet to investigate and encountered the being known as Trelane on Gothos. When Kirk, Sulu, Jaeger, McCoy, and DeSalle returned from the surface, Hadley left the bridge via the turbolift with Lieutenant Brent, Lieutenant Leslie, and an unnamed engineering crewman. Hadley was not on the bridge when Trelane suddenly appeared there a few minutes later. (TOS: "The Squire of Gothos")
From stardate 3087.8 to stardate 3088.7, Hadley wearing a red operations division uniform was part of the landing party that beamed down to an uncharted iron-silica planet after the universe, including the Enterprise, was rocked by a winking-out effect. This landing party discovered that Lazarus was responsible for the winking-out of the universe and later dealt with both Lazarus and anti-Lazarus. (TOS: "The Alternative Factor")
In 2267 on stadate 3142.8, Hadley was on the bridge when Khan Noonien Singh took over the Enterprise; he passed out after the life support to the bridge was cut, and was seated in a prisoner's row with Leslie, Brent, and others, forced to watch Khan's murderous spectacle on the viewscreen. (TOS: "Space Seed")
On stardate 3417.5, Hadley was assisting Dr. McCoy in helping to pack up and beam up the Omicron Ceti III colonist's possessions to have them relocate due to the deadly berthold rays when Lieutenant DeSalle introduced to everyone the pod plants. Just as with the rest of the crew, Hadley became infected with the spores and for the first and only time he mutinied and left the Enterprise. This was the only illness that Hadley was known to have suffered from during the Enterprise's original five-year mission. Hadley returned to duty on stardate 3417.7 once he and the rest of the crew were free of the spores' influence. (TOS: "This Side of Paradise")
On stardate 4202.9, Hadley, again at navigation, usually one to not fall (unlike most of the bridge crew) during attacks, took an unusual fall from navigation when the Enterprise engaged the planet killer and it attacked back. Hadley fell from navigation towards the railing of the bridge going down on the floor stomach and head first. Hadley had to pick himself up by kneeling. (TOS: "The Doomsday Machine")
On stardate 3615.4, Hadley was at navigation when Kirk ordered the whole crew to have tranquilizer's administered by hypospray to keep them calm when the Jack the Ripper entity took control of the Enterprise computer. Hadley was seen, for only a second time, laughing at Sulu's saying the entity sounded very gloomy. (TOS: "Wolf in the Fold")
In 2268 on stardate 4657.5, Hadley was on the bridge at the helm when Kelvan Hanar transported himself onto the bridge of the Enterprise. Along with the rest of the bridge crew, Hadley was temporarily put into "frozen" and motionless stasis by Hanar. The Kelvan Milky Way Expedition were attempting to hijack the Enterprise so they could return to the Andromeda Galaxy. On stardate 4658.9 as he was sitting at the Engineering Sub-Systems Checkout station, Hadley watched in horror as Kelvan leader Rojan neutralized and reduced Lieutenant Uhura and Ensign Chekov into inert solids. The Kelvan Drea then neutralized and reduced Hadley, an operations division ensign, and Leslie standing next to him, into a dehydrated porous cuboctahedron solid, the size of a Human fist, composed of his base minerals which represented the "distilled" essence of his being, along with much of the rest of the crew. The Kelvans considered Hadley one of many non-essential personnel. Hadley was reconstituted after Kirk, Spock, McCoy, and Scott, the only four of the crew not neutralized, regained control of the ship. (TOS: "By Any Other Name")
On stardate 5029.5, Hadley replaced Chekov at the navigation station when Tommy Starnes of the Starnes Exploration Party children, with mind control "powers" from Gorgan, planted a suggestion in Chekov's, Security Chief Freeman's, and another security guard's brains, to go to engineering to arrest Kirk and Spock on "orders" of Starfleet command. Hadley also fell under the influence of the telekinesis "powers" of the Starnes Exploration Party children, although it is unclear what suggestion they planted in Hadley's brain, although one could assume, correctly, it was something similar to what was planted in Sulu's brain of impending danger to the Enterprise if the ship did not continue to where the children and Gorgan wanted to go, Marcus XII. Hadley recovered from the mind-control and obeyed Kirk's orders once the children were freed from the influence of Gorgan. (TOS: "And the Children Shall Lead")
On stardate 5630.8, Hadley was present in the corridor when Kirk addressed "all ship's personnel", informing them that "clearance plans now in effect," and to "clear passageways immediately," as the recently arrived Medusan Ambassador Kollos was being escorting to his quarters. Later that evening, while leaving Deck 8, he was attacked by Larry Marvick, who became insane due to looking at the ambassador without wearing a visor, after he came out of the turbolift. Marvick gave Hadley a strong left hook punch into Hadley's right eye, temporarily knocking Hadley unconscious. Hadley quickly returned to the bridge, where he sat at the station between the science station and the viewscreen, opposite of the door, when the Enterprise was driven across the galactic barrier by Marvick. (TOS: "Is There in Truth No Beauty?")
Also in 2268, Hadley was attending Kirk's memorial service when Kirk was thought lost and possibly dead by the crew on the USS Defiant in the interphase of Tholian space. Hadley sat in the second row behind where Uhura and Sulu were sitting and the row across from an engineering crewman who suddenly went berserk due to the effects of the same interphase. The berserk crewman attacked Hadley and hit Hadley hard across his right shoulder blade making Hadley wince in pain. Fortunately, security guards pulled the crewman off of Hadley before the crewman could inflict any more pain. (TOS: "The Tholian Web")
Near the end of 2268 on stardate 5784.2, Hadley was at helm when the Enterprise was orbiting the planet Platonius when the ship suddenly started shaking violently. Hadley grimaced during this shaking but was able to keep from falling. Scott ordered Hadley to set the emergency gyros to maximum to try and stop the shaking but that didn't work. Fortunately, though, Dr. McCoy was able to get the shaking to stop on the planet Platonius by shaking Parmen to calm and later getting his fever down. (TOS: "Plato's Stepchildren")
Near the end of 2268, Hadley was again thrown violently from navigation backwards to the floor behind him onto his back when the Enterprise was hurled 990.7 light years due to the likeness of Losira defending the Kalandans outpost planet from the Enterprise. Hadley had to, again, right himself using his knees and hands. Fortunately for Hadley this was the second and final fall he suffered during the Enterprise's original five-year mission. Hadley was later as perplexed as Spock and Lieutenant Rahda when Scott, from engineering, informed the bridge that the ship "felt" wrong. (TOS: "That Which Survives")
On stardate 5730.6, Hadley attended a speech given by Lokai, in the recreation room, regarding that on his home planet of Cheron the black|right (white|left) order enslaved and continued to oppress the white|right (black|left) people that Lokai was a member of. On stardate 5730.7, Hadley, at the bridge's weapons sub-systems station, got a direct view of the brutal fight between Lokai and Commissioner Bele, as well as Kirk's ultimately futile attempt at ending the hatred of the two of them as the Enterprise reached the devastated and ultimately doomed Cheron. (TOS: "Let That Be Your Last Battlefield")
In 2269, on stardate 5832.6, Hadley relieved Ensign Pavel Chekov at navigation when Chekov was assigned to auxiliary control to help Spock search for the planet Eden that Dr. Sevrin and his followers were attempting to find. Hadley continued to relieve Chekov, at navigation, when Chekov went to his former love interest Irina Galliulin, who was one of the singers at the concert given by Sevrin's followers for the Enterprise crew. (TOS: "The Way to Eden")
- "The Corbomite Maneuver"
- "The Man Trap"
- "The Naked Time"
- "Charlie X"
- "The Galileo Seven"
- "Court Martial"
- "The Menagerie, Part I"
- "The Menagerie, Part II"
- "Shore Leave"
- "The Squire of Gothos"
- "The Alternative Factor"
- "Tomorrow is Yesterday"
- "The Return of the Archons"
- "A Taste of Armageddon"
- "Space Seed"
- "This Side of Paradise"
- "The Devil in the Dark"
- "The City on the Edge of Forever"
- "Operation -- Annihilate!"
- "Metamorphosis" (stock footage)
- "Friday's Child"
- "Who Mourns for Adonais?"
- "Amok Time"
- "The Doomsday Machine"
- "Wolf in the Fold"
- "The Changeling"
- "The Apple"
- "Mirror, Mirror"
- "The Trouble with Tribbles"
- "Bread and Circuses"
- "Journey to Babel"
- "A Private Little War"
- "The Gamesters of Triskelion"
- "The Immunity Syndrome"
- "A Piece of the Action"
- "By Any Other Name"
- "Return to Tomorrow"
- "Patterns of Force"
- "The Ultimate Computer"
- "The Omega Glory"
- "Assignment: Earth"
- "Spectre of the Gun"
- "Elaan of Troyius"
- "The Paradise Syndrome"
- "And the Children Shall Lead"
- "Spock's Brain"
- "Is There in Truth No Beauty?"
- "The Empath"
- "The Tholian Web"
- "Plato's Stepchildren"
- "Wink of an Eye"
- "That Which Survives"
- "Let That Be Your Last Battlefield"
- "Whom Gods Destroy"
- "The Lights of Zetar"
- "The Cloud Minders" (stock footage)
- "The Way to Eden"
- "Requiem for Methuselah"
- "The Savage Curtain"
- "Turnabout Intruder" (stock footage)
- "Trials and Tribble-ations" (archive footage)
Hadley was played by William Blackburn, a regular extra on Star Trek: The Original Series. Regarding the role, Blackburn stated, "I kept my mouth shut on the bridge for three years, after being told by the assistant director to just sit there. So I did." (Star Trek Magazine issue 116, p. 43) He elaborated about Hadley, "I did think of him as a character, but I thought of him as me. I played him like me, like what I would do in the same situation Hadley was in. It worked for me to do that. If anyone watches me, something that I always got good comments on was that I'm very stoic. I was very stoic playing the character, which is what I would have been if I was doing what he did. I've had people say to me, 'Well, you never smiled.' And I'd say, 'For God's sake, I'm sitting there at my job pushing buttons and navigating a ship with however many people on it.' I tried never to [overact] [....] So I was careful what I did with my face. I never wanted to overact, and you could really easily do that when you were acting without lines." 
The character of Hadley was not named until "A Piece of the Action" (while manning the science station in a gold command division tunic). William Blackburn appeared many times on the original series as this Starfleet officer, the most times of any extra. In wide shots of the Enterprise bridge, Hadley usually appeared, since many of the angle shots used were reuses of master shots filmed while Blackburn was on the set of TOS Season 1. (Sometimes, in a continuity gaffe, the back of Hadley's head appeared in wide shots, even though Chekov was supposedly sitting there, in later seasons). Hadley had lieutenant rank when he was in gold, but ensign when in red uniform. Starting with season 3, he was always an ensign, but still often seen wearing lieutenant's braid in stock footage.
Hadley was referred to as "lieutenant" on multiple occasions, but sometimes he wore an ensign's uniform (no insignia). His gold costume included the full lieutenant braid up until TOS Season 3, except for his brief appearance in "The Naked Time". It might be possible he was a lieutenant junior grade in the anomalous appearances, as this situation occurred with several other lieutenants in TOS, but speculation also existed the officer might have been demoted or only temporarily promoted.
In a deleted scene from "The Corbomite Maneuver", Hadley was shown as sitting at a bridge station behind Sulu, next to a seated Harrison, while Sulu related how, in his childhood, he had tried to emulate the facial expressions of Oriental villains from movies. ("Inside the Roddenberry Vault, Part I", Star Trek: The Original Series - The Roddenberry Vault special features)
Hadley was mentioned in Kevin Lauderdale's short story "Devices and Desires" from the anthology Constellations. In the story, Spock was contemplating a transfer from the Enterprise to a top secret Starfleet science station and told Kirk that Lieutenant Hadley would make an "excellent" science officer.
In the Star Trek Online mission "Return to Babel", Hadley is manning the helm.