After an emotional split, he realizes he has left some books in her apartment.Jerry tries to convince George that he does not need the books, as he has already read them, but George is nevertheless able to persuade Jerry to get them for him.After being informed, George informs Jerry he has no problem with him dating Marlene.The following night, Jerry asks Marlene to come to his apartment, but she tells him that it might be better not to date him anymore.
The episode contained a number of references to pop culture.
Elaine mentions that a man she knows used to nod at her whenever she saw him, but suddenly stopped, leading her to state, "[...] he went from nods to nothing." This prompts George to hum the Tony Bennett song "Rags to Riches", replacing the chorus with "nods to nothing".
He would frequently come up with the idea for an episode and make it into a teleplay with Seinfeld's help; in a 1991 interview with the Los Angeles Times, Seinfeld stated: "Most of the stories are from [David's] life. I mean, he just fills notebooks with ideas and I try to help him, but Larry is really the designer of the show." David Sims of The A. Club commented, "Seinfeld started its second season, its first real season after a four-episode test run, very strongly with "The Ex-Girlfriend", and it is the first time we really see George as the character we know and love, that weird dark shadow of Larry David's mind who behaves as no functioning human being honestly could." Among the actresses who auditioned for the part of Marlene were Amy Yasbeck, Jeri Ryan, who would go on to star in Star Trek: Voyager, and Heidi Swedberg.
Directed by Tom Cherones, "The Ex-Girlfriend" was the first episode of the show filmed at CBS Studio Center in Los Angeles, California (and would stay there for the remainder of the show's run), the previous season having been filmed at Desilu Cahuenga in Hollywood.
The episode featured one new set, a chiropractor's office; the remaining sets had been used on the show earlier.
One or two members of the crew shook the car to give the impression that it was moving, though it never actually was.