Joan Winsor Blos was born on December 9, 1928, in New York City, to Max and Charlotte Biber Winsor. She gained a high regard for the value and needs of children by observing her parents in their careers. Her father was a psychiatrist working closely with the problems of juvenile delinquents; her mother was a teacher and educator of teachers. Blos received a degree in physiology from Vassar College in 1950 and studied clinical psychology at the College of the City of New York, now City University of New York, earning a master's degree in 1956. She continued her studies in clinical psychology at Yale University, where she served as a research assistant in the Child Study Center. On June 7, 1953, she married Peter Blos, Jr., a psychoanalyst and researcher. They have two children.

Following her marriage, Blos reviewed children's books for the Connecticut Association of Mental Health from 1954 to 1956. She worked at the Bank Street College of Education from 1958 to 1970, serving as an associate in the publications division and teaching children's literature. She was also an editor for the Bank Street Readers and worked with the Writers' Laboratory. During this period Blos wrote several children's books: In The City (1964), People Read (1964), Joe Finds a Way (1967), and "It's Spring!" She Said (1968). She published Just Think! in 1971. From 1972 to 1980 Blos was a faculty member at the University of Michigan in Ann Arbor, giving lectures in children's literature for the School of Education. She served as the United States editor for the publication Children's Literature in Education from 1976 to 1981.

Blos continues to write books for young children, including Martin's Hats (1984) and Old Henry (1987), as well as historical fiction for young adults. She won the John Newbery Medal and the American Book Award in 1980 for A Gathering of Days, which was also designated as a Best Book of the Year by the School Library Journal.