He is a literary foil, juxtaposed to Leonard, and perhaps to a lesser degree Kurt, Penny's 'jock douche-bag' ex-boyfriend. Zack is always friendly, non-aggressive, and arguably even less intelligent than Kurt. Zack also appears to be tougher than an average person. He is presented as moderately attractive.
Zack, as well as his sister, are the Vice Presidents of a company that designs the Cheesecake Factory's menus (as well as other restaurant's menus) which is owned by his father and which is how he met Penny. In "The Lunar Excitation" (S03E23), he goes on a date with Penny who, allegedly due to having dated Leonard, finds his stupidity insufferable and winds up having a drunken sexual encounter with Leonard on the same night.
Zack makes a second appearance in "The Alien Parasite Hypothesis" (S04E10). He briefly meets Amy Farrah Fowler, who instantly feels sexually attracted towards him. She later approaches him at a bar hoping for sexual intercourse with him, but his stupidity becomes too much of a turnoff for her to go through with it. When they shake hands, Amy whimpers, saying that "should hold [her] over" for a while. Then, Amy says "Hoo".
Penny gets back together with Zack again in "The Justice League Recombination" (S04E11), later admitting to Leonard that she only did so in order not to be alone on New Year's Eve. In this episode Zack attempts to befriend Leonard, Sheldon, Raj and Howard. He dresses up as Superman for Stuart's New Year's Eve party, helping them complete their Justice League theme and win the contest for best group costume. He and Penny apparently broke up again sometime afterwards.
Zack appears again in "The Toast Derivation" (S04E17), having been invited to Sheldon and Leonard's apartment by Sheldon. This gathering is Sheldon's attempt to establish a new social group, along with Kripke, Stuart, and hopefully LeVar Burton, after Sheldon got angry because he was not the center of his previous social group. At the social event, Zack impresses Stuart and Kripke with his tale of a naked woman in a hot tub, and later the three end up singing karaoke, much to Sheldon's dismay.
In "The Thanksgiving Decoupling" (S07E09), Penny mentions the fake wedding ceremony she and Zack went through on a Thanksgiving trip to Las Vegas three years ago. Leonard, Sheldon and Amy explain that those wedding chapels are real and that Penny is legally married to Zack, who also thought the ceremonies were fake. Penny realizes that she is married and is Mrs. Zack Johnson. While dinner is being prepared, Penny has annulment papers downloaded, fills them out and calls Zack over to sign them. Dense as ever, Zack wonders if he should do it to their kids. Penny reminds him that they have no children and he asks if she's sure about that, since she didn't even know she was really married to him. He signs it and they all sit down to a Thanksgiving dinner.
In "The Cooper Extraction" (S07E11), Leonard makes up a story about what if Penny would have ended up with Zack. Leonard imagines that Zack would use the money for rent to buy magic beans, and then Zack and Penny would both pee their pants.
In "The Fermentation Bifurcation" (S9E22), Penny, Leonard, Howard, Raj and Amy see him again at the wine-tasting house and he takes an interest in Raj's newest companion, Claire. Raj sets him straight. Also, Zack wonders if the guidance system the guys are developing could be use by the military for war causing the guys worries. He also admits that he has always liked Leonard and Penny together, especially after seeing that they got married.
In "The Cognition Regeneration" (S10E22), Zack offered Penny a job, which makes Leonard less than thrilled because of the fact that it would be weird for exes to work together. Zack also announces that he has gotten engaged and that his fiancée doesn't want him working with Penny, so no job.
In "The Bitcoin Entanglement" (S11E09), Leonard and Penny visit him to reclaim Leonard's old laptop that Penny had given him years ago. Zack also shows them an old video of a drunk Penny made after she broke up with Leonard seven years before. She did love him, but was scared of a permanent commitment.
In "The Propagation Proposition" (S12E12), he meets the girls at a bar and buys them expensive champagne. He reveals he has sold his company (which he presumably inherited from his father) and has retired. He tells that he then got married and purchased a boat. He invited Leonard and Penny to join him and his wife on the boat. There he introduces his wife Marissa and the two surprise Leonard and Penny by explaining that Zack has discovered he is infertile. They wish to have a child and ask Leonard if he would consider being a sperm donor for them in exchange for a lot of money. After some disagreement, Leonard and Penny mutually decide to donate to them. The Hofstadters changed their minds in "The Donation Oscillation".
Zack reappeared in "The Donation Oscillation" only in the scene before the intro where he and Marissa visit Leonard and Penny in their apartment. After Leonard tells Zack that he can't go through with the donation, he then suggests that Sheldon do it, but Amy angrily says no when they ask him.
|The Big Bang Theory Episodes|
|Young Sheldon Episodes|
|Appeared||M = Mentioned||Did not appear||Not aired/Not available|
|V = Voiced||C = Cameo||F = Flashback||I = Indirectly mentioned||P = Pictured|
- Zack loves Milk Duds. Sheldon gives him a pack in "The Justice League Recombination" as an apology and he is seen eating them in "The Toast Derivation".
- Zack, much like Stuart, has been a love interest for both Penny and Amy, and, like Stuart, this has made him, at times, a rival for both Leonard and Sheldon. They are both tools for the plots. Except Zack and Leonard never showed any interest in Amy.
- In "The Fermentation Bifurcation", Zack tells Raj that he managed to get his muscular body through playing Marco Polo at a YMCA center.
- Like Dave Gibbs, Bert and Glenn, Zack is taller than Sheldon.
- Zack is friends with and an honorary member of The Social Group.
- Despite Zack's low intelligence, he still attended college.
- Aside from being Leonard's opposite (dumb, tall, and muscular), their interaction is also somewhat ironic, as Zack likes Leonard while Leonard hates Zack.