In addition to her novels and children's books, Duncan published several collections of poetry and nonfiction, including Who Killed My Daughter?

[4] After her daughter's murder, Duncan distanced herself from the thriller and horror genres, shifting her focus to picture books and novels aimed for young children.

[1] She spent her early life in Pennsylvania, relocating in her late childhood to Sarasota, Florida, where her parents resumed their employment as circus photographers.

[5] At age 15, Duncan was photographed by her father posed at Siesta Key, and the photo appeared on the cover of the July 9, 1949, issue of Collier's magazine.

[13] She published her first novel, Love Song for Joyce, in 1958 under the pen name Lois Kerry,[14] followed by Debutante Hill in 1959;[14] the latter was initially rejected for a literary prize because it featured an adolescent character drinking a beer.

[1] In 1962, Duncan moved to Albuquerque, New Mexico, with her children after divorcing her first husband, Joseph Cardozo, and supported herself writing greeting cards and fictional confessionals for pulp magazines.

[1] In 1966, she published the novel Ransom, detailing a group of students held captive on a school bus, which earned her an Edgar Allan Poe Award nomination, as well as marking her shift from romance to more suspense-oriented works.

[17] After the publication of I Know What You Did Last Summer, Duncan wrote Down a Dark Hall (1974), a Gothic novel following four students at an isolated and mysterious boarding school.

[19] In 1978, Duncan published the controversial Killing Mr. Griffin, a novel that details three high-school students' murder of their English teacher.

[20] Critic Margery Fisher noted Duncan's "unreserved" approach to writing the novel, in language she described as both "harsh and literal.

"[21] Killing Mr. Griffin was one of Duncan's major critical successes, and was selected as an American Library Association Best Book for Young Adults that year.

[30] Beginning in 2010, ten of Duncan's most successful teen novels were updated for a new generation and re-released in paperback with modern cover designs.

[17] On August 23, 2021, Albuquerque Police Chief Harold Medina announced that a suspect was picked up in July on unrelated charges.

"[27][40] As noted by Emily Langer of The Washington Post, Duncan often "plucked her characters from normalcy and placed them in extraordinary, often dark circumstances," in contrast to her contemporaries such as Beverly Cleary, Judy Blume, and Robert Cormier.

[39] The ALA Margaret A. Edwards Award recognizes one writer and a particular body of work for "significant and lasting contribution to young adult literature".