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. Gary Gilmore was executed for shooting two young men, Max Jensen and Ben Bushnell, in cold blood. He was executed by firing squad in Utah. Mikal Gilmore's 1995 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 USA Today comment that states the book is "one of the most beautifully written, moving nonfiction books published in the past five years." Gilmore's book 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.