Talented child performers, from Baby Peggy in the silent era to Emma Watson in the “Harry Potter” films, have long enjoyed a special relationship with moviegoers. One of the most captivating of these precocious luminaries was Hayley Mills, who starred in half a dozen Walt Disney films starting with “Pollyanna” (1960) and “The Parent Trap” (1961).

Ms. Mills, now 75, recollects her professional and personal lives as a child, adolescent and ingenue. “Forever Young” is a greatly enjoyable and eye-opening memoir: a show-business chronicle which proves equal parts celebration and cautionary tale. “I had some amazing luck and good fortune,” writes the still-working actress, “but it all came at a price.”

Born in Marylebone, London, Ms. Mills was the second of three children to noted stage and screen actor John Mills and actress-turned-writer Mary Hayley Bell. Clearly acting was in the Mills family’s blood. Hayley’s older sister, Juliet, began working onstage at 16 and within two years was nominated for a Tony Award.

Unlike her parents or sister, Hayley, nicknamed “Bags,” had no dramatic training except what she soaked up at home or backstage in theaters. But her childhood environment—thick with famous family friends who wrote, produced, acted and directed—set her on the path for professional duty. Her first movie role came at age 12 in “Tiger Bay” (1959), an independent English film in which she co-starred with her dad and newcomer Horst Buchholz. One newspaper reviewer declared: “Tiger Bay is dominated from first to last by Hayley Mills . . . who acts her father off the screen.” The youngster hid the notice under the sofa, so as not to hurt her parent’s feelings.

After the success of “Tiger Bay,” none other than Walt Disney himself flew to London to meet the Mills family and offer Hayley a lucrative seven-year contract. Her parents were rather floored by this, the author writes: “To them, I was still just Bags: a bit of a joke. They didn’t really think of me as an actress. . . . Did I even want to be an actress?” When negotiations stalled, Disney came up with a sweetener—one which Ms. Mills only learned of when researching this book. Leafing through her file at Disney Archives, she came across this memo: “If Hayley Mills signs with Disney, her father John Mills is guaranteed the lead role in [the 1960 film] Swiss Family Robinson.” The deal was sealed. Soon Hayley and her family were en route to the place her mother referred to with fear and loathing as “Horrorwood.”

But Hayley thrived in the California sunshine. “The contrast between gray London and boom-town Hollywood in 1959,” she writes, “was like the difference between East and West Berlin before the Wall came down.” Her lively manner and poignant essence charmed the camera and delighted audiences. “Pollyanna” was a global success, and Ms. Mills was awarded a “Juvenile Oscar” for her performance. Her second U.S. picture, “The Parent Trap,” starred “Hayley Mills and Hayley Mills” in the dual role of twins raised apart; her recording of a song from that movie was a hit in several countries. Other directors petitioned Disney to borrow his new contract player—notably Stanley Kubrick, who wanted her to play Lolita. (Disney said no.)

“So many remarkable things were happening,” Ms. Mills writes, “but I didn’t feel any ownership, because I wasn’t the one making the decisions. I was . . . trying to find myself and to mature, while my fame was growing exponentially.” At 14, she was getting over 7,000 fan letters a week—all addressed to a “Hayley Mills” who “was far prettier, wittier, and sexier than I actually was.” The “real” Hayley, she writes, was shy, insecure and even guilty about her success. “I simply didn’t know who I was anymore.” Away from the camera, “I’d start to withdraw into myself like a telescope.” At 16, she writes, “I felt like a total failure as a person.”

There were compensations. In 1964, she had a date with George Harrison of the Beatles: “He reminded me of a little foal,” she wrote in her diary, “peering out from under a bear skin rug.” In 1966’s “The Trouble With Angels,” she worked with legendary burlesque dancer Gypsy Rose Lee: “It was almost as extraordinary as meeting Mata Hari!” And she was earning a great deal of money, all of it sealed away for her in a trust not to be touched until her 21st birthday.

Yet her insecurity grew to an alarming degree, tipping into paranoia. Working in “The Family Way” (1966), she writes: “I started to seriously doubt whether they were even bothering to put film in the camera when they shot my close-ups . . . because they thought I was so awful they wouldn’t be able to use it.” She even made an attempt at suicide while driving, closing her eyes and crashing into a hedge.

But seeing the Rev. Billy Graham speak in London in 1966, and meeting him afterward, proved an “inspiring” experience: “That evening . . . was the beginning of my spiritual search.” Meeting another Graham, the English-born journalist Sheilah, was also a “life-changing event.” Sheilah Graham was F. Scott Fitzgerald’s Hollywood companion, and she shared with Hayley the “College of One,” a reading syllabus Fitzgerald devised for Graham’s self-instruction. This “set me on the road to my own personal education,” Ms. Mills writes. “Books have been a passion and a solace ever since.”

She would need solace in the coming years, what with a problematic 6-year marriage to filmmaker Roy Boulting (32 years her senior), further career disappointments (though there were also successes) and an excruciating development regarding that supposedly foolproof trust fund.

The persistent Ms. Mills soldiered on, discovering she had, in addition to acting ability, writing talent. “I wonder,” she suggests, “whether this book is perhaps my first real chance to understand . . . the strange and remarkable things that happened to me.” “Forever Young” is an unexpected treat: a brave and revealing memoir with nary a dull passage.

Mr. Nolan reviews crime fiction for the Journal.