Duncan was born Lois Duncan Steinmetz on April 28, 1934 in Philadelphia, Pennsylvania, the first child of Lois Duncan (née Foley) and Joseph Janney Steinmetz.
Duncan had one younger brother, William Janney "Billy" Steinmetz.
Both of Duncan's parents were professional magazine photographers who took photos for the Ringling Bros. and Barnum & Bailey Circus.

She spent her early life in Pennsylvania, relocating in her late childhood to Sarasota, Florida, where her parents resumed their employment as circus photographers.
In Florida, she spent her youth among circus performers, including The Doll Family.
Her experience growing up in this environment would eventually serve as the basis of her picture books The Circus Comes Home (1993) and Song of the Circus (2002).

Duncan described herself as a "shy, fat little girl," a "bookworm and dreamer" who spent her childhood playing in the woods.
Duncan cited The Princess and the Goblin and The Wizard of Oz and Mary Poppins series among her favorite novels as a child.
She started writing and submitting manuscripts to magazines at age ten, and sold her first story at the age of thirteen.
At age fifteen, 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.

She graduated from Sarasota High School in 1952.
The following autumn, she enrolled at Duke University, but dropped out in 1953 to start a family with Joseph Cardozo, a fellow student she had met at the university.
Duncan was born Lois Duncan Steinmetz on April 28, 1934 in Philadelphia, Pennsylvania, the first child of Lois Duncan (née Foley) and Joseph Janney Steinmetz. Duncan had one younger brother, William Janney "Billy" Steinmetz. Both of Duncan's parents were professional magazine photographers who took photos for the Ringling Bros. and Barnum & Bailey Circus.
She spent her early life in Pennsylvania, relocating in her late childhood to Sarasota, Florida, where her parents resumed their employment as circus photographers. In Florida, she spent her youth among circus performers, including The Doll Family. Her experience growing up in this environment would eventually serve as the basis of her picture books The Circus Comes Home (1993) and Song of the Circus (2002).
Duncan described herself as a "shy, fat little girl," a "bookworm and dreamer" who spent her childhood playing in the woods. Duncan cited The Princess and the Goblin and The Wizard of Oz and Mary Poppins series among her favorite novels as a child. She started writing and submitting manuscripts to magazines at age ten, and sold her first story at the age of thirteen. At age fifteen, 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.
She graduated from Sarasota High School in 1952. The following autumn, she enrolled at Duke University, but dropped out in 1953 to start a family with Joseph Cardozo, a fellow student she had met at the university.
Lois Duncan