The drama's final episode will air on Sept. 18.
The Guinness Book of World Records has cited it as the longest-running television drama. It began as a 15-minute serial on NBC Radio on Jan. 25, 1937, and debuted on CBS TV in 1952, focusing on the fictional town of Springfield and the Spaulding, Lewis and Cooper families.
"For many of us, it was the first show we ever watched," said Lynn Leahey, editorial director of Soap Opera Digest. Unlike prime-time shows that came and went, it was a constant in people's lives. "It really is heartbreaking to see something like this go away."
Procter & Gamble Productions, which makes the show, informed cast and crew Wednesday at separate sets in New York and New Jersey. The company isn't giving up on the story, and will explore different ways to keep it going after September, a spokeswoman said.
Soap operas have been in a long, slow decline in popularity, primarily because many of the women who made up their loyal audiences are no longer at home at that hour. They're working, and can find the communal experience that their favorite soaps once gave them elsewhere.
"Guiding Light" had the lowest ratings of the eight daytime dramas on the air. When it leaves, CBS and ABC will have three weekday soap operas each, and NBC will have one.
"The numbers are really tough for all of these old dramas," said Ron Raines, the actor who portrayed the villain Alan Spaulding on "Guiding Light."
"I don't think any of the other shows want any of us to go off. We're all in this together," he said.
Many successful actors got their start on "Guiding Light," including James Earl Jones, Calista Flockhart, Hayden Panettiere, Kevin Bacon and Taye Diggs.
Faced with extinction a year ago, "Guiding Light" significantly revamped its operations. It ditched its fixed, three-camera set in favor of portable cameras that enabled producers to shoot in different locations. The move saved money and changed the show's look to make it seem more like the reality shows younger viewers are accustomed to.
It didn't work, at least enough for CBS. The network hasn't said what will replace "Guiding Light" on the schedule, but it will almost certainly be a talk or game show, which are much cheaper to make than dramas with a large cast.
The changes also made many of the fans and cast members unhappy, said Carolyn Hinsey, Soap Opera Digest columnist. Two of its biggest stars, Beth Ehlers and Ricky Paull Goldin, quit and now work on ABC's "All My Children."
For fans of the genre, Wednesday's move could be a peek into the future.
Ten years from now, "I absolutely think (daytime dramas) will still be around," Leahey said. "I don't know if you'll be able to watch them from noon to three o'clock on network television."
In fact, the cancellation could be an opportunity for "Guiding Light," she said. Perhaps there's a way to keep the show alive on cable or online; Procter & Gamble says it will have to evaluate whether there's a cost-effective way to do that.
For now, its cast and crew are in mourning.
"What is it? 72 years continuous?" Raines said. "That will never be touched. It's a very sad thing, but these are the times we live in. It's very tough out there."