Gilmore was born on February 9, 1951, in Portland, Oregon, to Frank and Bessie Gilmore.
In 1977, Gilmore's brother Gary, a convicted murderer, was the first person executed after the death penalty was reinstated in 1977. Mikal Gilmore's 1994 memoir, Shot in the Heart, details his relationship with Gary and their often troubled family, starting with the original Mormon settlers and continuing through to Gary's execution and its aftermath. Shot in the Heart received positive reviews, including a comment by New York Times critic Michiko Kakutani calling the book "[r]emarkable, astonishing...Shot in the Heart reads like a combination of Brothers Karamazov and a series of Johnny Cash ballads...chilling, heartbreaking, and alarming." In 1994 Shot in the Heart won the Los Angeles Times Book Prize and the National Book Critics Circle Award.
Gilmore was long interested in music, and in the early 1970s began writing articles for Rolling Stone. In 1999, Gilmore's chronology Night Beat: A Shadow History of Rock and Roll was published by Anchor. In July 2009 Gilmore released another book, Stories Done: Writings on the 1960s and its Discontents. It was published by Free Press.