“Mamma Mia!” is still going strong 20 years after its April 6, 1999, debut at London’s Prince Edward Theatre. The longevity is a testament to the band ABBA and to the persistence of producer Judy Craymer, director Phyllida Lloyd and writer Catherine Johnson. The stage musical opened with low expectations; in 1983, another tribute to the Swedish quartet, named “Abbacadabra,” closed eight weeks after its debut. Plus, “Mamma Mia!” had been in workshops for 18 months, with Johnson the third writer on the project (she did seven drafts).

Eight months after the debut, Variety reported that the show was “a financial gusher,” making back its £3 million ($4.8 million) cost in 27 weeks, and boasting an advance of nearly $13 million. The stage musical has earned an estimated $4 billion, and that’s not counting income from the two films. When it opened, Lloyd said: “We hope to create pure pleasure. We’re not splitting the atom.”

That sense of self-mockery was part of the show’s appeal, along with the great pop songs and a finale that inevitably brings audiences to their feet.

On the 10th anniversary of the stage show, Variety declared it “a global phenomenon,” adding, “It’s the show that gives jukebox musicals a good name.” Others had tried the jukebox format with varying degrees of success. But “Mamma Mia!” not only brought in crowds, it brought in people for repeat viewings.

The plot overlaps with the 1968 comedy “Buona Sera, Mrs. Campbell.” That movie concerns a single mom (Gina Lollobrigida) who’s been lying to her grown daughter about the identity of the father — because the woman had three suitors and honestly doesn’t know which one is the actual parent.

Craymer, Lloyd and Johnson repeated their duties on the 2008 film, which was a gamble. None had any big-screen credits, and the box office had been underwhelming for film adaptations of stage hits like “Rent” and “The Producers.” But “Mamma Mia” earned more than $600 million, or roughly 10 times its budget. It was one of the biggest hits of Meryl Streep’s career.

Universal executive Donna Langley laughed, “I convinced myself we were making a Bollywood movie. That helped me sleep at night. Bollywood movies have a wonderful, celebratory, melodramatic feel to them.”

The 2018 sequel cost a little more to make and was still a big hit, with nearly $400 million at the box office.