A "camp" gay man, Jack has always had a passion for acting and the theatre, although his friends, amongst others, doubt his abilities in this area. Very mercurial, he changes careers rapidly from cater waiter, Banana Republic and Barneys New York sales clerk, to acting teacher, to student nurse, to surfer, to back-up dancer for Jennifer Lopez and Janet Jackson. Although he considers all his relationships serious, only one lasted longer than a few weeks (with Stuart, a client of Will's played by Dave Foley), and even that one ended with Jack's cheating.
Jack is unashamedly vain and self-absorbed, with an adoration of all gay icons, particularly Cher, of whom he has a rare doll. (He met her once, although he mistook her for a drag queen and declared, "You're not that great, Mr. Sister. I do a better Cher than you." It was only when she slapped him and exclaimed "Snap out of it!" that he realized and promptly fainted.) He would also meet Cher in a dream where she appeared as God (complete with an entourage of "dancing fairies"). When Jack asked her if she was God, Cher replied, "That depends on what bath house you pray at." He also collects clippings of celebrities' hair, including a complete collection from the four main actresses on The Golden Girls, as well as Bernadette Peters, Betty Buckley, Idina Menzel and — as of season seven — Patti LuPone. He is also decidedly hyperactive and air-headed, causing Will to once refer to him as "Jumping Ferret Jack," and another time thinking that he had set Will and himself up on dates with twins, but soon realizing it was just one guy taking his glasses on and off. Strangely, he believes in gay rights, but absolutely hates lesbians (as gay men are stereotypically believed to), to the point of having harassed their friends Terry and Annie ("Starsky and Butch?!" Jack exclaims) during practice for a gay-sensitivity skit, requested that their parts be played by men, and only referred to them as "the lesbians" the whole time, even to their faces. His general feelings about gay rights seem to be shallow at times, like when a kiss between two men on a television show he liked was not shown, he asked Will "What about our Constitutional right to see two hotties get it on?" (to which Will replied "Would that be the Constitution that begins 'Wheeeee, the people'?"), but his intentions are essentially good.
Jack was born in February 1969, and was raised by his mother Judith (Veronica Cartwright) and stepfather Daniel (Beau Bridges). His mother did not know he was gay ("How could she not know? What is she, headless?" Karen once exclaimed), and his stepfather was not close to him, however it is clear that he, unlike Judith was aware that Jack was gay, having been told while in his teens. According to Jack, he passed out, at which point Jack had to perform CPR on him ("Of course that might just have confused him further," Jack later muses). He claims to have come out as early as preschool and has made up assorted stories about his youth claiming this, leaving into question which are truth and which are not. While he was in high school, Jack befriended college student Will Truman (Eric McCormack) and helped Will realize his own homosexuality. They have been best friends ever since. Although Jack often made his Stepfather sound cold and unloving, he is actually very friendly and gets along well with Elliot, much to Jack's annoyance.
When the series begins, Jack's home is unknown but he is living with Will while his floors are re-done. At the start of season two however, Karen refers to him as having been living 'above a blinking light in Times Square'. At the end of season one, Jack agrees to marry Karen's maid Rosario (Shelley Morrison) in order to secure her green card. Jack lives with Karen for the entirety of season two but after a year together, the marriage is annulled when Rosario wants to marry Karen's gardener. Therefore at the end of season two, Jack is homeless and moving in with Will, but when Will takes in his other best friend Grace Adler (Debra Messing) after an attempted burglary at her apartment, Jack agrees to sublet Grace's apartment — although Will, Karen, and his mother pay the rent. (In "Moveable Feast Part 1," it emerged that each of the three pays one half of the cost, meaning that Jack is actually collecting one and a half times the actual amount of his rent.)
Since the first season, Jack has established a close bond with alcoholic socialite Karen Walker (Megan Mullally), who chooses to work merely as a source of distraction. Karen is his closest female friend (much like Grace is to Will), and they bond due to their outlandish, self-absorbed behavior. Jack also strikes up a good friendship with Grace, through association with Will and Karen.
In season three, Jack undertakes a search for his biological father, which ends when he discovers that his father died several years back. Jack does, however, meet Elliot (Michael Angarano), his own biological son, whom he fathered by depositing at a sperm bank as a teenager so that he could buy a coat. He later discovers that the boy's mother Bonnie (Rosie O'Donnell) is a lesbian; she was a nurse at the sperm bank who stole Jack's deposit and was inseminated with it.
Jack's most consistent vocation results from his taking over the class of his acting teacher Zandra (Eileen Brennan) after she is fed up with his lack of talent. After taking everyone's money and leaving, another student (played by Stacy Keach) takes over the class and begins teaching the "McFarland method" ("acting is attracting") without Jack's knowledge. He trains to be a student nurse, but ultimately gives it up when, during Karen's fourth wedding, he meets Jennifer Lopez and becomes her back-up dancer (and, for a short time, Janet Jackson's back-up dancer), a career that fizzles as well.
In the seventh season, Jack embarks on a proper career as a producer for OutTV, a new gay television network. By season's end, his profile rises to the point that he is given a talk show of his own to host.
In season eight, Jack loses his job at the network after he speaks out against the new right-wing ownership of the show, which has given him a supposedly conservative co-host named Amber Louise (played by Britney Spears) who is, by her own admission, a hard-core lesbian. He has difficulties finding work and out of desperation, auditions for a minor role on a television show. Feeling depressed and rejected at the time, he doesn't rely on his over-dramatic acting style and ends up getting the lead role of Chuck Rafferty, a straight, alcoholic, womanizing cop.
In the show's series finale, Jack had a brief and unwanted relationship with Karen's diminutive millionaire friend and social rival, Beverley Leslie, who left Jack as his sole heir in the case of his death. Beverley then had the misfortune to be blown off his penthouse balcony by a gust of wind, and Jack was left with millions of dollars. Decades pass and Jack is then shown to have lived with Karen (who had lost her money when she found out everything Stan owned was borrowed) and Rosario in Karen's mansion for the rest of his life after the show ended.
Jack is known for his several various flings throughout the show. Most of them are mentioned but not shown, and Jack never seems to take any of them seriously or settle down with anyone for very long. Jack's longest relationship is with Stuart Lamarack (Dave Foley), which lasts several months during the sixth season, until Stuart cheats on Jack.
In the episode Lows in the Mid-Eighties, it is also shown that Jack had (and may still have) feelings for Will. In the flashback, he confessed his love to Will, but tried to passed it off as a 'test of friendship' when Will didn't feel the same.
Jack is also the owner, or former owner, of a dog (Klaus Von Puppy) and a parrot (Guapo). Since the start of season three until he moves into Karen's mansion at some point after the eighth season, Jack lives across the hall from Will and Grace, and in his spare time he operates a one-table café, Jacques, in the hallway outside his front door.