The script originally was entitled Guardian Angel and had been written by Deutsch as a vehicle for Myrna Loy and William Powell a dozen years earlier. Later, Katharine Hepburn and Spencer Tracy were set to be in the same scenario, but it eventually fell through. When Lucy and Desi Arnaz expressed interest in another film, the studio, Metro Goldwyn Mayer, dug this script out of their unused screenplay archives for the comedy couple's new scenario for the film.
The Metro-Goldwyn-Mayer release was filmed in part on location in Yosemite National Park. Interiors were shot at the Desilu Studios in Culver City, California. It was the first time Desilu was involved in feature film production.
Forever, Darling was the second film made by Lucille Ball and Desi Arnaz during hiatus from their weekly CBS television sitcom I Love Lucy, following The Long, Long Trailer in 1954. The couple's marriage was showing signs of severe strain, and Ball hoped the project would bring them closer together. They promoted the film via a cross-country train tour aboard a special car provided by the Santa Fe Railroad, with stops in Chicago, Detroit, Dallas, Cleveland, Pittsburgh, Philadelphia, New York City, and Ball's hometown of Jamestown, New York.
Cary Grant was Ball and Arnaz's original choice for the Guardian Angel; however, he demanded a far higher salary than they were willing to pay. James Mason was then sought and hired for the salary that Grant had rejected. I Love Lucy writers Madelyn Pugh and Bob Carroll, Jr. were called in to help salvage what Arnaz felt was a weak script. Their uncredited contribution was a lengthy slapstick camping sequence that had little to do with the plot that preceded it. The film was considered "sub-standard" by programmers at Radio City Music Hall, where Trailer had premiered, and it opened instead at the Loew's State Theatre, where the newlywed couple had performed their first vaudeville act in 1941. It was a critical and commercial flop that barely recouped its $1.4 million cost. As a result, MGM opted out of its agreement for a two-picture deal with Desilu, and Arnaz decided to forgo plans to create a feature-film division at his studio.
The title song, with lyrics by Sammy Cahn and music by Bronislau Kaper, was recorded by both Arnaz and the Ames Brothers, who performed it over the opening credits and ultimately had the bigger hit. The tune became an Arnaz family tradition, sung by Desi at special events, including his daughter Lucie's marriage to actor Laurence Luckinbill.