She wrote more than 100 books, including the best-selling and award-winning I Spy series for children, written completely in rhythm and rhyme.

[2] Marzollo was a high school English teacher in Arlington, MA (1965–1966) and the assistant director of Harvard's Project Upward Bound (1967).

In New York City, she worked on early childhood research projects for General Learning Corp. (1967–1969) and was the Director of Publications for the National Commission on Resources for Youth (1970–1971).

For 20 years (1972–1992), she edited Scholastic's "Let's Find Out" Magazine, working closely with art director Carol Devine Carson.

[2] Among her other children's books are: Help Me Learn Addition, Help Me Learn Numbers 0-20, The Little Plant Doctor, Pierre the Penguin, Mama Mama/Papa Papa, Close Your Eyes, Pretend You're a Cat, Happy Birthday, Martin Luther King, and the Shanna Show books.