Jack Geller is the father of Ross Geller and Monica Geller and husband to Judy Geller. He is best known for his ill-timed, inappropriate comments, such as addressing Emily's father while in London as a "thieving, would-be-speaking-German-if-it-weren't-for-us, cheap little man". He often tries to get himself out of these things using the line 'I'm just saying...' He led his in-laws to believe that he's an attorney, although his line of work was revealed to be in the Military. He secretly smoked cigarettes, claiming that the cigarettes Ross found around the house were Judy's. His inappropriate comments often were based on sexual themes, such as when he started discussing Monica and Chandler's sexual strategies for getting pregnant.
Although both Jack and Judy were criticized for favoring Ross over Monica, Jack was the more sympathetic of the two. He constantly referred to Monica as Harmonica. When Monica broke up with serious boyfriend Richard Burke, he consoled her, although he had previously disapproved of her dating his long-time best friend, a man twenty-one years her senior. He also gave her his Porsche, after all objects of her childhood memories were ruined by a flood. In The Last One, Monica names her new baby son after her father this being against the Jewish custom. He had admitted he felt like a father when newly born Ross held Jack's finger.
Jack and his wife, Judy love each other very much, therefore they are a very happily married couple.
Ross Geller and Monica Geller
Like his wife, Jack favors Ross over Monica because of his lovable nature over Monica's silly behavior. However, unlike Judy, Jack is more sympathetic towards her than she is: He told her to ignore her mother's cynical comments, consoled her after her break-up with Richard, and gave her his Porsche after her childhood things were destroyed by a flood.
Jack has a good relationship with Rachel as she's his daughter's best friend. They have a relationship of mutual respect and friendliness and they both grow closer to being related when Rachel has Emma and Jack becomes her paternal grandfather due to Jack's son being Emma's father. It is hinted that Jack becomes Rachel's father-in-law and Rachel becomes Jack's daughter-in-law when it's hinted that Ross and Rachel had remarried.
Jack and Chandler first met in 1987 where he joined Ross at the Gellers' house for Thanksgiving dinner. They didn't fully interact again until "The One Where Ross Got High" where he and Judy come to dinner at Monica's apartment and he is not quite fully supportive of him as they thought he smoked marijuana in college. When Chandler and Monica discover that Ross was the one who got high and Blamed it on Chandler, Ross tries to avoid telling them but seeing that Ross won't Say anything, Monica coerces him into telling the truth after she tells her parents. With the truth out, Jack changes his mind and he and Judy take a liking to Chandler and thank him for standing by Ross during his "drug problems" and for "taking on Monica as well".
- Jack never appeared inside any of his son's legal apartments.
- Jack and Judy were the only parents of any of the main characters to appear in all 10 seasons of the show.
- It appears that Jack is the reason Ross was not comfortable about having a male nanny in "The One With The Male Nanny".
- The One With The Sonogram At The End (First appearance)
- The One Where Nana Dies Twice
- The One With Two Parts, Part 2
- The One With The List (Appears in photograph)
- The One With The Lesbian Wedding (Appears in photograph in uncut version)
- The One With The Prom Video
- The One Where Joey Moves Out
- The One Where Eddie Won't Go (Appears in photograph)
- The One With The Princess Leia Fantasy
- The One With The Race Car Bed (Vaguely appears in photograph)
- The One Where Rachel Quits (Vaguely appears in photograph)
- The One Where Monica And Richard Are Friends (Mentioned only)
- The One With Phoebe's Ex-Partner (Vaguely appears in photograph)
- The One Where Ross And Rachel Take A Break (Vaguely appears in photograph)
- The One With The Morning After (Vaguely appears in photograph)
- The One With The Tiny T-Shirt (Vaguely appears in photograph)
- The One With The Dollhouse (Mentioned only) (Vaguely appears in photograph)
- The One With The Cuffs (Mentioned only)
- The One With Joey's New Girlfriend (Mentioned only)
- The One With The Invitation (Appears in flashback)
- The One With Ross's Wedding, Part 1 (Mentioned only)
- The One With Ross's Wedding, Part 2
- The One After Ross Says Rachel
- The One With All The Thanksgivings (Appears in flashback(s))
- The One Where Ross Got High
- The One With Rachel's Book
- The One With Phoebe's Cookies
- The One Where Rosita Dies
- The One Where They All Turn Thirty (Appears in flashback(s))
- The One With Rachel's Big Kiss (Mentioned only)
- The One With Monica And Chandler's Wedding, Part 1
- The One With Monica And Chandler's Wedding, Part 2
- The One After 'I Do'
- The One In Massapequa
- The One Where Rachel Has A Baby, Part 1 (Mentioned only)
- The One Where No One Proposes
- The One With The Male Nanny (Mentioned only)
- The One With Ross' Inappropriate Song (Mentioned only)
- The One With Rachel's Other Sister (Mentioned only)
- The One With The Cake
- The One Where Joey Speaks French (Mentioned only)
- The Last One, Part 1 (Mentioned only)