Trelease was born in Orange, New Jersey to George Edward and Jane (Conlan) Trelease. In 1963 he married Susan Kelleher; the couple has two children: Elizabeth Jane and James Joseph, Jr. Trelease received a BA from the University of Massachusetts at Amherst in 1963. He served in United States Army Intelligence from 1964-1966 as a First Lieutenant.Trelease lectures to school groups and educational gatherings across the nation, often in conjunction with purveyors of books for young people, about the fundamental importance of youthful reading to the entire process of education.