Zack Johnson  is an on-and-off boyfriend of Penny's who first appeared in "The Lunar Excitation" (S03E23). He and Penny once got married on a whim while in Las Vegas; Penny was unaware it was a real wedding and ended up having it annulled later. Though usually a friendly, outgoing person, Zack is extremely dimwitted and displays even less understanding of sarcasm than Sheldon does. Despite his stupidity, he is quite interested in science.
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. He is in between Leonard and Kurt in all perceivable aspects; he has a fairly remedial and unremarkable career (unlike Leonard), but has no mentioned criminal record (unlike Kurt). Zack also appears to be weaker than Kurt, but the same height as Kurt as well as tougher than an average person. He is presented as moderately attractive.
Zack, as well as his sister, is the Vice President of the 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 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 explains 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.
- 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.
- 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, Zack is taller than Sheldon.