Billy Wilder originally planned to cast Paul Douglas as Sheldrake, but after Douglas died unexpectedly, Fred MacMurray took over the role.

The initial concept was inspired by Noël Coward's Brief Encounter, but Wilder had been unable to make the adultery comedy he envisioned in the 1940s due to Hays Code restrictions.

The film was partly based on a real Hollywood scandal in which producer Walter Wanger shot agent Jennings Lang for having an affair with Wanger's wife, actress Joan Bennett, during which Lang had used a low-level employee's apartment for trysts.

Another plot element was inspired by the real-life experience of a friend of co-writer I.A.L. Diamond, who came home after a breakup to find his girlfriend had committed suicide in his bed.

Although Wilder usually required actors to stick exactly to the script, he let Jack Lemmon improvise the nasal spray scene and the spaghetti-cooking scene where he strains pasta through a tennis racket while singing.

In a scene where Lemmon was supposed to only mime being punched, he failed to move correctly and was accidentally knocked down for real; Wilder kept this take in the final film.

Lemmon actually caught a cold, as scripted for his character, after a park bench scene was filmed on a bitterly cold autumn night.

Art director Alexandre Trauner used forced perspective to build the large insurance office set, employing successively smaller people and desks, down to children, to exaggerate its size; he also furnished Baxter's deliberately small, shabby apartment with thrift-store items and some of Wilder's own furniture.

The film's title theme, composed by Charles Williams and originally titled 'Jealous Lover', had first appeared in the 1949 film The Romantic Age; a later recording by Ferrante & Teicher as 'Theme from The Apartment' reached #10 on the Billboard Hot 100 in 1960.

After the film's release, Fred MacMurray, known for playing guileless characters, was accosted by women in the street who berated him for making a 'dirty filthy movie,' and one even hit him with her purse.

Kevin Spacey dedicated his Best Actor Oscar for American Beauty (1999) to Jack Lemmon's performance in The Apartment, and American Beauty director Sam Mendes had watched The Apartment as inspiration while preparing to shoot his film.

Since The Apartment won Best Picture in 1960, only two black-and-white films have won that Oscar since: Schindler's List (1993) and The Artist (2011).