Folger was born in San Francisco. Her parents were Peter Folger, Chairman and President of the Folger Coffee Company, and Ines “Pui” Mejia (1907–2007), the youngest child of Gertrude and Encarnacion Mejia, a consul general of El Salvador. She had a younger brother, Peter, Jr. (born 1945). Her Roman Catholic parents divorced in 1952 when she was nine years old, after her mother ended the marriage on the grounds of extreme cruelty. In 1960, her father married again, this time to his 24-year-old private secretary, Beverly Mater, who was already pregnant with his youngest daughter, Elizabeth, born in January 1961.  Growing up in San Francisco, Folger was interested in art, books, poetry and playing the piano. Close friends and family called her “Gibbie”.

Folger attended Santa Catalina School in Monterey, California, near Carmel.[3] She graduated with honors in June 1961. She then matriculated at Radcliffe College in Cambridge, Massachusetts in the fall of 1961. During her time at Radcliffe, Folger became an active member of the college’s Gilbert and Sullivan Players, a musical theatre group. She starred in two of its productions, starting with The Sorcerer in April 1963 where she played the part of one of the town’s villagers. In December 1963, she starred in The Gondoliers as one of the Contadine. She graduated with honors from Radcliffe in 1964.  While a freshman in college, Folger held her coming out party on December 21, 1961 at the St. Francis Hotel in San Francisco, where she made her official debut into San Francisco’s high society. Her debutante ball was one of the highlights of the social season, with Folger wearing a bright yellow Christian Dior gown that she had purchased in Paris the previous summer.  After graduating from Radcliffe, she enrolled in the fall of 1964 at Harvard University, also in Cambridge, where she did graduate work and received a degree in Art History. After graduating in the spring of 1967, Folger took a job at the University of California Art Museum in Berkeley, California as a publicity director. While employed there, her main job was to organize the fine art museum council.

In September 1967, Folger decided to leave California in order to find herself and to probe the other side of life. She soon made the move to New York City, where she got a job working for a magazine publisher. She eventually left for a job at the Gotham Book Mart on 47th Street. While living in New York, she lived well below her means. Folger’s annual income from her inheritances, after taxes, was $130,000 a year (the equivalent of $838,193 a year in 2009 dollars).

In December 1967, Folger met Polish author Jerzy Kosinski at a bookstore party. Kosinski was married to American steel heiress Mary Hayward Weir. Mary traveled in the same wealthy circle as Folger, and it was she who introduced her husband to Folger. In early January 1968, Jerzy Kosinski introduced Folger to his friend, aspiring writer Wojciech Frykowski, at a party and the two liked each other. Frykowski had only been living in the United States for about a month. At the time, Frykowski was not then fluent in English, but, like Folger, he was fluent in French. She gave him a tour of New York, began to teach him English, and they fell in love. He moved into her New York City apartment and she supported him financially.

In August 1968, both Folger and Frykowski decided to move to Los Angeles. He wanted to pursue his writing career while Folger wanted to get involved with a new welfare project that was currently under way. She rented a car, and she and Frykowski drove across the country.  In Los Angeles, she found a two-story hilltop home to rent at 2774 Woodstock Road for her and Frykowski in Laurel Canyon, and bought a 1968 yellow Firebird. Their neighbor across the road was singer Cass Elliot of the musical group The Mamas & the Papas, whom the couple quickly befriended. Through Frykowski, Folger met director Roman Polanski and his wife, actress Sharon Tate. Folger and Frykowski were later introduced to Jay Sebring through Tate and Polanski. The five quickly began to hang out together and were known to be a part of “the beautiful people crowd” in Hollywood. Folger later invested in Jay’s hair salon empire, as she agreed to purchase around $3,500 worth of stock in Sebring International.

On April 1, 1969, while Roman Polanski was away in Europe filming movies, Folger and Frykowski moved into the Polanskis’ Cielo Drive home in Benedict Canyon, at Polanski’s request. At the same time, their Woodstock Road home was being occupied by Frykowski’s friend, Polish artist Witold-K, who had arrived in the United States the previous December. A day earlier, Folger had quit her job as a volunteer. It was around this time that Folger’s relationship with Frykowski began to deteriorate, and she considered leaving him. Constantly fighting, the pair sank lower into the world of drugs which had long passed the experimental stage.

In May 1969, Folger and her mother Ines attended the San Francisco opening of Jay Sebring’s newest shop at 629 Commercial Street. Folger enjoyed the champagne reception and found herself mingling with such guests as Paul Newman, Joanne Woodward, Doris Tate, and her husband Paul (Sharon Tate’s parents).  On July 8, 1969, Folger and Frykowski learned that Sharon Tate would be returning to the U.S. later that month. The couple then began to move most of their clothing from Cielo Drive back to their own home on Woodstock Road. They informed Frykowski’s friend, Witold-K, that they would be soon returning to live in their home after Tate’s arrival.  On July 20, 1969, Tate returned to California from London and asked Folger and Frykowski to remain at 10050 Cielo Drive with her until her husband Roman Polanski arrived on August 12. Folger, Frykowski, Tate and Sebring, along with Tate’s parents and two younger sisters, all watched the moon landing on television.  On August 6, 1969 film director Michael Sarne invited Folger, Frykowski and Tate over for a dinner party at his rented Malibu beach house. After dessert had been served, Tate began to feel unwell, so it was decided that Folger and Frykowski would drive her home.

On August 8, 1969, Folger and Frykowski ran some errands together. Folger purchased a yellow, lightweight bicycle around 2 p.m. from a shop on Santa Monica Boulevard and arranged for it to be delivered to Cielo Drive later that afternoon. She and Frykowski then drove back to the Polanski home and had a late lunch with Sharon Tate and her friends, Joanna Pettet and Barbara Lewis, on the front lawn patio. The late lunch was served to the party of five by Winifred Chapman, the Polanskis’ housekeeper. Shortly after, at around 3:45 p.m., Folger left Cielo Drive in her Firebird in order to keep an appointment she had later that afternoon. Frykowski left minutes later, in Tate’s rented 1969 red Camaro, to unload a box at the couple’s Woodstock Road home.

That evening, just after 9 p.m., Folger, Wojciech Frykowski, Jay Sebring, and Sharon Tate went out to a Mexican restaurant called El Coyote. Returning home, Frykowski fell asleep on the couch while Folger was in her room reading. Her mother called her at 10 p.m. that night to verify their weekend plans. She was scheduled to fly to San Francisco at 10 a.m. Saturday morning on United Airlines in order to celebrate her birthday.

During the early morning hours of August 9, three of Charles Manson’s followers: Susan Atkins, Charles “Tex” Watson, and Patricia Krenwinkel, broke into the Cielo Drive home and killed Folger, Sharon Tate, Jay Sebring and Wojciech Frykowski. Steven Parent, an 18-year old friend of the property’s caretaker William Garrettson, was also killed when he encountered the group in the driveway of the home.

When investigators were called in later that morning, the scene which greeted them was unlike anything they had ever witnessed. On the lawn lay the bodies of Folger and Wojciech Frykowski, separated by just a few feet. Folger had been stabbed twenty-eight times and died from a stab wound to the aorta. Although the coroner found the drug MDA in Folger’s system, they reached the conclusion that she was fully aware of what was happening when the attack occurred. Frykowski had been shot twice, struck in the head thirteen times with a blunt object, and stabbed a total of fifty-one times. Tate had been stabbed sixteen times, and Sebring had been shot twice and stabbed seven times.  During her trial, Patricia Krenwinkle admitted that she had chased Folger onto the lawn and stabbed her. Patricia said Folger told her to stop stabbing her stating, “I’m already dead.”  Folger’s body was returned to San Francisco and taken to Crippen and Flynn Mortuary in Redwood City. Her funeral was held on the morning of August 13, 1969, at Our Lady of the Wayside Church in Portola Valley, a church that had been built by her grandparents, the Mejias, in 1912. Following a Catholic Requiem Mass, Folger was entombed at Holy Cross Cemetery in Colma, California.[16] Folger was survived by her parents, Peter and Ines, her brother, Peter, Jr. then 24, and sister, Elizabeth, then 8.  After her death, investigators reported that Folger’s estate was worth $530,000. She left no will.