Born in Erie, Pennsylvania, DeCecco studied at Allegheny College, where he took his degree in biology in 1946. He continued his studies at the Unversity of Pennsylvania, where he received master's and doctor's degrees in English and Modern European History. He then taught history at the University of Detroit. At the same time he pursued post-doctoral studies in psychology at Wayne State University and later at Michigan State University. Eventually he taught there from 1955 to 1960, when he left to escape the widespread purges of "homosexuals".
John was a member of the Psychology Department at San Francisco State University (SFSU) from 1960 to 2007; he also taught at Columbia University. Author of about ten books, De Cecco is perhaps best known as the editor of the long-running Journal of Homosexuality, a post he has held since 1977. He remains a magnet for controversy, recently criticized for trying to publish an essay, contributed by psychologist Bruce Rind, about inter-generational sexual relationships. "Religious lobbies don't want this published or even investigated," he said. "People are in denial of the less acceptable."
Recently De Cecoo has had his papers archived at the Gay, Lesbian, Bi and Transgender Historical Society. Entitled the "John Paul De Cecco Papers," the extensive collection, donated by De Cecco in June 2001, chronicles decades of LGBT-related research, which he and his colleagues conducted at San Francisco State University. These historical artifacts, which contain hundreds of sex-related surveys, interviews, and in-depth research files from the late 1970s and early 1980s, offer comprehensive data on a broad range of topics from homosexual couples and lesbians over 60 to gay discrimination and male-male sexual assault from an era when mainstream psychiatric literature still described homosexuality as deviant behavior or mental illness. "There really wasn't a sense of gay community or identity until the last half of the century," De Cecco said. "These papers are part of that story and give a record of what gay identity meant in the period collected." Inspired by the gay liberation movement of the late 1960s, De Cecco's work also documents many of his professional milestones, such as co-founding and directing SFSU's program in human sexual studies in 1981, his groundbreaking Variations in Human Sexuality course, and numerous awards.