Winsor was born October 16, 1919 in Olivia, Minnesota but raised in Berkeley, California. At the age of 18, Winsor made a list of her goals for life. Among those was her hope to write a best-selling novel. Winsor graduated in 1938 from the University of California, Berkeley. During her school years, she married a fellow student, All-American college football player Robert Herwig. In 1937, she began writing a thrice-weekly sports column for the Oakland Tribune. Although that job only lasted a year, Winsor later returned to the newspaper to work as a receptionist. She was fired in 1938 when the newspaper chose to trim their workforce.

Winsor became interested in the Restoration period through her husband. Herwig was writing a paper for school on Charles II, and, out of boredom, Winsor read one of his research books.

Her husband joined the military at the outbreak of World War II and spent five years with the US Marines fighting in the Pacific theatre. During that time, Winsor studied the Restoration period, claiming to have read 356 books on the subject. She began writing a novel based on her research. Her fifth draft of the novel was accepted for publication. The publishers promptly edited the book down to one-fifth of its original size. The resulting novel, Forever Amber, was 972 pages long. A thousand-page saga, the novel frolicked through Restoration England and vivid images of fashion, politics, mild sex scenes, affairs and public disasters of the time, including the plague and the Great Fire of London.

While many reviewers "praised the story for its relevance, comparing Amber's fortitude during the plague and fire to that of the women who held hearth and home together through the blitzes of World War II", others condemned it for its blatant sexual references. Fourteen U.S. states banned the book as pornography. The first was Massachusetts, whose attorney general cited 70 references to sexual intercourse, 39 illegitimate pregnancies, 7 abortions, and "10 descriptions of women undressing in front of men" as reasons for banning the novel. The Hays Office also condemned the work, but within a month the movie rights had been purchased by Twentieth Century Fox. The film, directed by Otto Preminger and starring Linda Darnell and Cornell Wilde, was released 1947. Winsor denied that her book was particularly daring, and said that she had no interest in explicit scenes. "I wrote only two sexy passages," she remarked, "and my publishers took both of them out. They put in ellipses instead. In those days, you know, you could solve everything with an ellipse."

Despite its banning, Forever Amber was the bestselling US novel of the 1940s. It sold over 100,000 copies in its first week of release, and went on to sell over three million copies.

Made a celebrity by the success of her novel, Winsor found it unthinkable to return to the married life she had known with Herwig and, in 1946, they divorced. Ten days later, she became the sixth wife of the big-band leader and clarinetist Artie Shaw, despite the fact that two years previously Shaw had castigated his then-wife, Ava Gardner for reading such a "trashy novel" as Forever Amber. The marriage to Shaw ended in 1948, and Winsor soon married her divorce attorney, Arnold Krakower. That marriage likewise ended in divorce, in 1953. In 1956 Winsor married for the fourth time, to Paul A. Porter, a former head of the Federal Communications Commission. Their marriage ended in 1975 with his death.

Winsor's next commercially successful novel, Star Money, appeared in 1950, and was a portrait closely drawn from her experience of becoming a bestselling author. But in five subsequent novels, the last appearing in 1986 -- The Lovers, Calais, Robert and Arabella, Jacintha, and Wanderers Eastward, Wanderers West -- she failed to make as much of an impact.

In 2000 a new edition of Forever Amber was published with a foreword by Barbara Taylor Bradford.

Winsor died May 26, 2003 in New York City.