Fanning was born Diane Lynn Butcher in Baltimore, Maryland. She attended Perry Hall High School, then Lynchburg College in Virginia, where she majored in chemistry.
After college, she wrote for the advertising field, earning more than 70 Addy Awards for her work. During that time, she also wrote magazine articles and personal essays as a freelance writer.
Her career shifted into nonprofit work with a move to New Braunfels, Texas. Fanning worked for fundraising groups, including Another Way Texas Shares and the National Association for Choice in Giving. She began her first book while living in Texas and eventually quit her nonprofit day job to write full time. In addition to writing books, she is co-founder of and a regular contributor to Women in Crime Ink, described by the Wall Street Journal as "a blog worth reading."
One of her titles, Written in Blood, received an Edgar Award nomination.
In 2002, Fanning corresponded with serial killer Tommy Lynn Sells, who, in a letter to Fanning, confessed to murdering 10-year-old Joel Kirkpatrick, whose mother was convicted of killing her son. Fanning's testimony before a prison review board about her letter from Sells, according to the Innocence Project, helped land Harper a new trial and, ultimately, an acquittal. Fanning's book, Through the Window, which details Sells' crime spree, was also said to help.
Fanning has been interviewed for national TV news shows, including CBS's "48 Hours Mystery" in November 2009. She regularly makes speaking appearances, including, as examples, the Texas Book Festival and, locally, the Blanco (Texas) Woman’s Club. In November 2009, 48 Hours' "Crimesider" column featured her in a story about the Casey Anthony case.
Diane and Wayne Fanning have three children: Pete Fanning, Ivy Johnson and Liz Nichols.