She moved to Rome, Georgia at the age of seven and graduated from Darlington School in 1982. She received a BA in religious studies at the University of North Carolina in 1986 and an MFA in creative writing at Boston University a year later. She has taught at various schools, including Cape Fear Community College (1990—1993), University of North Carolina at Chapel Hill (1995—1996), University of New Mexico (1998—2001), and Shorter College (2002—2008). Currently, she is an assistant professor of English at Kennesaw State University.

Other than her novels, her fiction and nonfiction works have appeared in many journals, anthologies and magazines, including Story, Boulevard, The New Yorker, New Stories from the South, Voices of the Xiled, Atlanta Magazine, Five Points, Ladies Home Journal, and Kennesaw Review.

She has lived in different places throughout her life. As a Peace Corps volunteer, she taught English in Senegal, West Africa, from 1988 to 1990. She also lived in New Mexico, Alaska, and Provincetown. Shortly before the publication of The School of Beauty and Charm, she moved back to Rome, Georgia due to an illness in her family.

In 2010, she won a National Endowment for the Arts Literary Fellowship. She received the Whiting Writers' Award for her first novel, Polite Society. Upon the publication of her second novel, The School of Beauty and Charm, in 2001, she was named the “Artist of the Year” by Rome Area Arts Council of Rome, Georgia.

In her first novel, Polite Society, Darren, a young woman from Tennessee, explains her experiences in Senegal as a Peace Corps volunteer.

Her second book, The School of Beauty and Charm, Sumner portrays a girl names Louise Peppers and her wild adolescent life. Raised in an affluent, Christian-oriented Southern family, Louise struggles under the pressure from her parents to become a “proper young lady” while she gets herself involved in alcohol and drugs.

Her third novel, The Ghost of Milagro Creek, is expected to be published in July 2010 by Algonquin.

The ghost of a curandera called Abuela narrates the tale of "The Ghost of Milagro Creek", a novel set in the austere beauty of northern New Mexico. With the help of curandismo and a gift for magic, the orphaned narrator has escaped from an Indian Boarding School and eked out a life in the woods with her younger brother, Ernesto. By the early eighties, when Ernesto is the deputy of Taos, the middle-aged Abuela has another mission — saving her grandson, Mister Romero, from his abusive mother. Skirting the watchful eye of Ernesto, Abuela performs a burial ceremony to save the soul of four-year-old Mister. After this rebirth, Mister lives in Abuela’s adobe house beside Milagro Creek, just across the bridge from the hard-drinking Mondragon family. When Mister and his best friend Tomas Mondragon are teenagers, they both fall in love with the pretty and precocious gringo, Rocky O’Brien. Abuela’s death coincides with Rocky’s departure to a nearby monastery, leaving Mister and Tomas, now in their early twenties, bereft with grief. On Good Friday, the young men meet on Milagro Creek Bridge with two stolen glocks to perform a double-suicide. When one of the guns does not go off, the survivor must pick up the pieces of love and betrayal, guilt and redemption.

Anthologies