Born in Brazil, and moving to the United States as an infant, she first came to prominence as a teenage model, as well as for her spokesperson work for CoverGirl cosmetics, which began in 1963 and spanned three decades.

O'Neill's breakthrough role came in Robert Mulligan's period drama Summer of '42 (1971), in which she portrayed the wife of an army serviceman during World War II, who becomes the subject of a teenage boy's romantic attraction.

She has since authored several books, including a memoir, Surviving Myself (1999), in which she detailed her career, marriages, experiences with anxiety and postpartum depression, and her religious faith.

[4] O'Neill's father, born in Puerto Rico,[5] was a bomber pilot in World War II, and later owned a medical supply company.

"[3] When she was an infant, she relocated with her family to the United States,[8] where she and her older brother Michael were raised in New Rochelle, New York, and Wilton, Connecticut.

At age 14, after her parents informed her the family was relocating to New York City, O'Neill attempted suicide by overdosing on sleeping pills,[10] and fell into a coma for approximately two weeks.

"[8] After her family's relocation to New York City, two of O'Neill's neighbors suggested that she model: "That appealed to me, because then I could buy my own horse and no one could take anything away from me again.

"[8] By age 15, while attending the prestigious Dalton School in Manhattan, she began appearing on the covers of Vogue, Cosmopolitan, and Seventeen, earning $80,000 ($806,000 today) in 1962.

"[1] O'Neill largely used her modeling income to fund her equestrian endeavors,[1] which afforded her to purchase her own horse, named Alezon.

[9] O'Neill eventually dropped out of the Dalton School at age 17 to wed her first husband, IBM executive Deed Rossiter.

[19] O'Neill next had a leading role in the psychological horror film The Reincarnation of Peter Proud (1975), co-starring with Michael Sarrazin and Margot Kidder, and directed by J. Lee Thompson.

[21] By the mid-1970s, O'Neill had forged a career in Italy, first starring in Luigi Zampa's drama The Flower in His Mouth (1975) opposite James Mason, which was shot on location in Sicily.

[26] O'Neill starred opposite David Carradine in the aviation-themed drama Cloud Dancer (1980), followed by a lead role in David Cronenberg's science fiction horror film Scanners (1981), portraying a woman who leads an oppositional group against a malevolent private military company creating biokinetic and psychokinetic humans.

[28] She starred in NBC's short-lived 1982 prime time soap opera Bare Essence and played the lead female role on the 1984 television series Cover Up.

On October 12, 1984, Jon-Erik Hexum, O'Neill's co-star in the Cover Up television series, mortally wounded himself on the show's set, unaware that a gun loaded with a blank cartridge could still cause extreme damage from the effect of expanding powder gases.

In 1991, O'Neill starred in the thriller film Committed, portraying a nurse who discovers the fellow staff at the psychiatric hospital she has been hired at are in fact inmates.

[32] After the birth of her first child, Aimee, O'Neill experienced undiagnosed postpartum depression and committed herself to a psychiatric hospital for treatment, which included electroshock therapy.

[6][8] O'Neill initially disbelieved the accusation after Lederer passed several lie detector tests, and the abuse allegation strained her relationship with her daughter.

'[6] In her 1999 autobiography Surviving Myself, O'Neill describes many of her life experiences, including her marriages, career, and her move to her Tennessee farm in the late 1990s.

[40]O'Neill continues to be active as a writer working on her second autobiography, CoverStory, an inspirational speaker, and fundraiser for the benefit of crisis pregnancy centers across the United States.

She also founded Hope & Healing at Hillenglade, an equine therapy foundation in Nashville, Tennessee that serves war veterans suffering from post-traumatic stress disorder.