Jack was the unofficial leader of a group of genetically-engineered Humans who lived with him under psychiatric care by Dr. Karen Loews. Jack's parents took him, as a child, to undergo an illegal procedure called accelerated critical neural pathway formation which dramatically increased his intelligence beyond what Humans would call genius level. Jack also had superior physical abilities, such as enhanced agility. However, the procedure also turned Jack into a violent, anti-social, aggressive man who went on to have serious problems fitting into mainstream Federation society. Jack was taken in to psychiatric care and lived with others like himself.
In 2374, after it was revealed that Doctor Julian Bashir had undergone the same procedure, Doctor Karen Loews - the doctor in charge of Jack's case - took Jack, along with Patrick, Lauren, and Sarina Douglas to Deep Space 9. Dr. Loews believed that meeting someone like them who had managed to fit in to mainstream society would be helpful to them. Jack, however, objected, pointing out that Bashir only fit in because he hid his abilities and pretended to be "normal". Jack even threatened Sarina's life at one point.
Along with the others, Jack studied Gul Damar's speech calling for peace talks between the Dominion and the Federation. Together, just by watching the speech, they were able to determine the entire story of Damar's rise to power without ever having heard any of it before. Bashir was very impressed, and encouraged their thirst for knowledge about the Dominion. They asked to watch the holographic recordings of the peace talks, and determined that the Dominion was stalling for time to solve their shortages of ketracel-white. However, they also pointed out that, should the Federation refuse to sign the treaty, the Dominion would lose their only chance to secure a new supply of ketracel-white and, given the inevitable shortage, would have no choice but to send all the Jem'Hadar to their deaths in a devastating all-out suicide strike against the Federation.
Grateful to them, Starfleet provided Jack and the others with classified reports and war plans. They quickly determined that the Dominion was certain to win the war and that the only way to prevent the loss of billions of lives was to surrender immediately. Unsurprisingly, Captain Benjamin Sisko rejected this immediately. Jack, Patrick, and Lauren attempted to take Starfleet's classified information directly to the Dominion, knocking Bashir out and leaving him in Sarina's custody, so they could win the war more quickly, thus saving lives. Jack miscalculated, however, as Sarina released Bashir, who then stopped him from meeting with Weyoun. Starfleet, however, remained willing to listen to any ideas they had on how to win the war. (DS9: "Statistical Probabilities")
In 2375, Julian Bashir developed a procedure that would cure Sarina of her inability to properly communicate with others. With Patrick impersonating a Starfleet admiral, Jack and the others went to Deep Space 9, where their help proved invaluable for Bashir to perform the procedure successfully.
Jack also had fanciful ideas about preventing what he saw as the inevitable collapse of the universe in around sixty trillion years. He believed that by altering the cosmological constant via the use of a vast number of subspace field generators placed throughout the universe, this collapse could be avoided. (DS9: "Chrysalis")
"You know why they brought us here, don't you? Why they carted us halfway across the quadrant? They're going to experiment on us."
"Stop it, Jack."
"They want to find out what makes our genetically engineered brains tick. They're going to cut our heads open and see what comes out!"
"He was genetically enhanced when he was a boy, just like all of you."
"No, no. He's not like us. I never saw him at the Institute. He wasn't locked away for being too smart. He's passed himself off as normal. He's Mister Normal Starfleet man. Mister Productive Member of Society. Well, maybe we can learn to be just like him. Wear little uniforms. Yes, sir. No, sir. Thank you, sir."
"I'm not trotting anything out. All I'm saying is there's a reason we've been barred from certain professions. But that doesn't mean we can't be productive members of society."
"Here it comes. The we can still contribute speech. No. No, no, no, no. I will not forget what was done to me. I will not be part of a society that put me away for being too smart."
"Methought I heard a voice cry, Sleep no more! Damar does murder sleep!"
"Genetically engineered mutants. Can't you tell?"
"The fact is that the universe is going to stop expanding and it is going to collapse in on itself. We've got to do something before it's too late."
"How much time do we have left?"
"Sixty trillion years, seventy at the most."
Jack was played by actor Tim Ransom.
The script for "Statistical Probabilities" describes Jack as, "...an agitated man of about thirty-five, pacing back and forth and ranting half to himself. He's JACK, and as we'll learn, he's a maladjusted product of twenty-fourth century genetic engineering." This would put his year of birth around 2339. 
The characters of Jack, Patrick, Lauren and Sarina were collectively referred to as the "Jack-pack" by the Deep Space Nine writing team, with each character having a specific personality trait. Jack, for example, was thought to have had noteworthy literary roots. Indeed, the inspiration for Jack's character came from a popular work of fiction. "In the break session, we thought of the Jack character as Neal Cassady, the hero of Jack Kerouac's novel On the Road who talks a mile a minute and quotes philosophy by heart," explained staff writer David Weddle. "The final character that René [Echevarria] developed is quite far from that, but that's one of the basic ideas he started from." (Star Trek: Deep Space Nine Companion, p.513)
Actor Tim Ransom had previously auditioned for a number of roles in Star Trek unsuccessfully, including that of Doctor Bashir. When auditioning for the role of Jack, he put in a lot of effort to get it right. "The producers had told me that Jack was a bit manic, so I figured he's the equivalent of a guy who drinks forty cups of coffee a day." Ransom recalled. "I brought that energy into the room and I think that's what got me the job." (Star Trek: Deep Space Nine Companion, p.514)
During rehearsals, Ransom had issues with his costume. "Those [tight pants], man!" he laughed. "It was hard enough to stand up straight let alone to do all of that bending around and other stuff. And I didn't do the backflip. I wish I could lay claim to that and I hate to destroy the illusion, but that was a stunt double." (Star Trek: Deep Space Nine Companion, p.514)
The initial idea for "Chrysalis" was for it to be an episode centered around Jack. "It was going to be a 'Jack' show, where we'd do something like Flowers for Algernon," Echevarria explained, referring to Daniel Keyes's story about a mentally-disabled man who is turned into a genius via a scientific experiment, only to ultimately revert. "Jack would become normal. And we tried for days to break this story. It boiled down to the fact that it was a tragedy that this guy becomes normal, which kind of glamorizes mental illness. That's a very common sort of Hollywood story. 'Oh, aren't they cute - don't rob them of their originality and make them normal.' Which is bull. So we were stymied. Time was running out and we were just sitting there. And then all of a sudden Hans [Beimler] said, 'What if it's about Sarina? A love story with her.' And boom, that was it." (Star Trek: Deep Space Nine Companion, p.613-614)
The musical scene in "Crysalis" posed a problem for tone-deaf Ransom, so much so that his lines had to be dubbed over in post-production. As he later recalled, "Faith and Hilary and Michael are singers. I, however, am barely comfortable singing in the shower! So that was a frustrating shooting day - but not as frustrating as the looping session when we actually tried to lay the tracks in. The other three had enough talent to do it, so I'm the only one on there who had to be dubbed. René [Echevarria] is a good friend of mine, but I curse his name for writing that scene. That was so frigging hard!" (Star Trek: Deep Space Nine Companion, p.615)
In his review of "Statistical Probabilities", Star Trek author Keith R.A. DeCandido enjoyed Ransom's portrayal of Jack's manic affect, particularly praising his body language, however felt that the "scary" side of the character didn't come across very well. "There’s a disconnect between how he’s written — as menacing and dangerous — and how he’s performed," DeCandido wrote. "I honestly had totally forgotten that he was supposed to be scary and dangerous — notably seen in when he deliberately injures Lowes — because Ransom doesn’t really sell that element of his character. Still, he sells the rest of it superbly..." 
Jack appears in a number of non-canon works.
In the Pocket DS9 novel A Stitch in Time, Jack visits Elim Garak's shop and tells him he plans to persuade Starfleet to appoint Bashir commander of Deep Space 9, feeling a genetically enhanced person should be leading the station during the war. Jack and the others petition the Federation Council, but no one takes them seriously.
In the Star Trek: Section 31 novel Abyss, set in 2376, Section 31 Agent Cole tells Bashir that many genetically enhanced people living in the Federation would consider Jack to be a "social butterfly" compared to them. Cole also predicts that if the rogue genetically engineered 31 agent Ethan Locken starts a war, Starfleet would abandon Jack and the others.
As written in the Star Trek: Deep Space Nine - Mission Gamma novel Cathedral, Doctor Bashir becomes affected by the cathedral/anatheme artifact which causes him to experience a vision whereby he is placed in a cell with Jack and the others. Jack subsequently helps Bashir to escape.
Jack also has a card in the Star Trek Customizable Card Game, which describes him as a "Maladjusted misfit".