(coexisting on another dimensional plane)
Spock was the Terran/Vulcan hybrid first officer of the ISS Enterprise in 2267. He was a logical being who was loyal to his captain, James T. Kirk, and chose to warn him when he was ordered by the Terran Empire to kill him and take command if he did not launch an attack against the Halkans in retribution for their refusal to negotiate with the Empire.
Based on his captain's apparently erratic behavior, Spock correctly deduced that Kirk had actually been accidentally replaced by his counterpart from a mirror universe, and devised a means of returning Kirk, Scotty, McCoy, and Uhura to their original ship.
Before Kirk left, believing that Spock would one day become captain of the Enterprise, he planted a seed of doubt about the inevitability of the Empire's collapse, asking Spock if violence was the only logical answer. Spock, as logical as his counterpart, was aware of the Halkan prediction that the Empire's subject peoples would revolt in approximately 240 years, and the Empire's defeat was a foregone conclusion. Kirk submitted that it was illogical of Spock to continue to serve an Empire destined to collapse, but Spock said the odds of one man bringing about peaceful change to the Empire were virtually insurmountable. Kirk revealed the existence of the Tantalus field, an assassination device that the mirror universe Kirk had used to eliminate his enemies, and urged Spock to ally the device's power with his own formidable intellect. Spock appeared intrigued, and promised to consider Kirk's words. (TOS: "Mirror, Mirror")
As Kirk predicted, Spock later on became the captain of the Enterprise. Spock used the ship as a power base to accumulate influence, and eventually rose to become commander in chief of the Terran Empire. He instituted major reforms, turning the Empire into a more peaceful and less aggressive power. Unfortunately, Spock's reforms left the Empire unprepared to fight the united Klingon-Cardassian Alliance, who conquered the entire Terran Empire, enslaving the Terrans themselves as well as the Vulcans. (DS9: "Crossover")
Mirror Spock was portrayed by Leonard Nimoy, who also portrayed the prime universe Spock. Both characters were identical, except that mirror Spock famously wore a goatee beard (Van Dyke beard does not connect chin to moustache): the "evil twin" imagery evoked by mirror Spock's beard would later enter the larger cultural lexicon as a result of the character's portrayal in this episode.
On featuring the mirror-universe Spock in Star Trek: Deep Space Nine, Ronald D. Moore commented, "We were quite intrigued by the notion of using the Mirror Spock, but it never worked out for various reasons". In 1999, rumors circulated that Nimoy would appear in "The Emperor's New Cloak" as Spock. ("Looking Ahead", Star Trek Monthly issue 50)
Nimoy's portrayal of mirror Spock in "Mirror, Mirror" was the inspiration behind the name for the progressive-metal group "Spock's Beard".
In the Star Trek: The Next Generation novel Dark Mirror by Diane Duane, Jean-Luc Picard, masquerading as his mirror universe counterpart aboard the other's USS Enterprise-D, briefly investigates Spock's history after his encounter with the primary universe Kirk. According to the official history, Spock succeeded, briefly, in becoming a powerful figure in Starfleet and the Empire, thanks to his own cunning and the use of the Tantalus Field, but his advocacy of less warlike policies eventually made him too many enemies. Both Spock and his father were discredited, then convicted of treason and executed. As the novel predates the establishment of the history of the televised mirror universe, these events are contradicted by the history related by Intendant Kira to her counterpart in "Crossover", as well as the appearance of the mirror universe's Sarek in DIS: "The Wolf Inside".
Spock appears in the novella "The Sorrows of Empire", which was subsequently expanded into the full length novel of the same name. Several days after the events of "Mirror, Mirror", Spock is depicted as murdering the mirror Kirk and thereby assumes the captaincy of the ISS Enterprise. He later marries Marlena. The story focuses on Spock's rise to the leadership of the Terran Empire in 2277, his rule and the events leading to its transformation into the short-lived Terran Republic and the subsequent conquest of Earth and Vulcan by the Alliance in 2295.