Born Teresa Ann Waller in Van Nuys, California, in 1949, she lived in the Los Angeles area until the age of 14, when her family moved to northern California. In high school, she spent her lunch hours in the school library, devouring one historical novel after another. At about that same time she discovered fantasy writers T. H. White, J. R. R. Tolkien, and C. S. Lewis, and science fiction writer Andre Norton, whose combined influence would inspire her to begin creating imaginary worlds of her own. She met her husband, John Edgerton, in 1971, when she was working as a Tarot reader at a local Renaissance faire, and he was part of a Society for Creative Anachronism troupe putting on demonstrations of medieval tourney combat. They have four grown children: Gwyneth, Daisy, Megan, and John-Miles.
Her interest in medieval society, alchemy, magic, and Celtic mythology led her to begin writing the Green Lion Trilogy, a project which was to consume the next ten years of her life. The first book of the series, Child of Saturn, was a finalist for the Compton Crook Award. It is set in Celydonn, a fictional Celtic realm whose carefully-worked out tongue is, in the writer's own words, "not intended to represent any Real-world celtic tongue of dialect" but rather is "an intentionally fanciful combination of Welsh, Irish and pseudo-Celtic elements" resulting from "a pattern pf migrations, intermarriages and inter-tribal cultural exchanges which could have taken place on Ynis Celydonn and nowhere else".
After completing this trilogy, her interest turned from epic fantasy to the pseudo-sciences of the 18th century, which resulted in the steampunkish fantasy of manners Goblin Moon, a book which still commands an enthusiastic cult following. Other books and short stories followed.
However, publication of The Queen’s Necklace - despite good reviews - didn't produce enough sales to satisfy her publishers, and for a time she was unable to renew her publishing contract. In 2004 she was offered the chance to relaunch under the pseudonym Madeline Howard, and has since struck out with a new trilogy, Rune of the Unmaking, with the successful first novel, The Hidden Stars.
Notable themes in Edgerton's work include the struggle to maintain decency, dignity, and kindness even when faced with dire circumstances. Her heroes and heroines are often conflicted characters who chastize themselves harshly for failings that her readers easily forgive. Edgerton’s prose is richly descriptive, reflecting her love of 19th and early 20th century writers, and her desire to construct a textured and realistic secondary reality for each of her works.