McDermott was born in Brooklyn, New York. She attended St. Boniface School in Elmont, New York, on Long Island (1967), Sacred Heart Academy in Hempstead (1971), and the State University of New York at Oswego, receiving her BA in 1975, and received her MA from the University of New Hampshire in 1978.
She has taught at UCSD and American University, has been a writer-in-residence at Lynchburg College and Hollins College in Virginia, and was lecturer in English at the University of New Hampshire. Her short stories have appeared in Ms., Redbook, Mademoiselle, The New Yorker and Seventeen. She has also published articles in The New York Times and The Washington Post.
Ms. McDermott lives outside Washington, D.C. with her husband, a neuroscientist, and three children. She is Catholic, though she once deemed herself "not a very good Catholic