Allegra Huston was born Allegra Soma in London, England.

From birth until age four, she lived in London with her mother, Ricki, and a nanny; in January 1969, Ricki died in a car crash. Shortly thereafter, Allegra was introduced to John Huston and was told that he was her father. She stayed briefly with a friend of her mother's so she could complete the school year, and then went to live with her father in Ireland. During this time she was introduced to Danny Huston, whom she thought was her half-brother.

During one summer, she visited her Soma grandparents in Long Island, New York, and got to know her aunts, uncles and cousins. A little later, she lived with her grandparents for about a year.

She then lived in Mexico and, shortly after Huston's marriage to Celeste Shane, moved to L.A.

Allegra spent several years in L.A., living, at various times, with one of her father's friends; with her stepmother, Celeste, and her son; and with presumed sister Anjelica Huston, and her boyfriends Ryan O'Neal and Jack Nicholson.

During this time, Allegra learned that her biological father wasn't John Huston but was actually John Julius Cooper, 2nd Viscount Norwich; she has two half-siblings through him and his first wife.

Allegra graduated from high school at 15 and spent the subsequent summer as an assistant on a movie her father was filming in Europe. After production wrapped, she went to London to see her half-brother Tony Huston. She later attended and graduated from Oxford, then worked in London for several years, maintaining connections to her many relatives.

In the mid 1990s, she moved back to the U.S. and became an acquisitions-and-development consultant for Pathé Pictures. During this time, she met Cisco Guevara; they had a son, Rafael Patrick Gerónimo Niño de Ortíz Ladrón de Guevara, in 2002.

In 2007, she wrote an autobiography titled "Love Child: A Memoir of Family Lost and Found."