As a struggling writer and cartoonist in his late twenties, Graysmith spent much of his time studying, writing, editing, and conversing at the Owl & the Monkey cafe in the Inner Sunset district of San Francisco. He made significant contributions to Marvel Comics' Crazy Magazine, including the "Just Plain Folks" stories of Steve Gerber.

Graysmith is best-known for his writings on the Zodiac Killer case. He first became involved in the case when he was a cartoonist for the San Francisco Chronicle. In his book Serial Killers and Their Victims, Graysmith blames the Zodiac murders on one time suspect Arthur Leigh Allen even though his handwriting, fingerprints, physical description and DNA do not match. Graysmith based his book and case on actual police files. He was also disciplined for plagiarism whilst at the Chronicle and has been suspected to be the author of the fake 1978 Zodiac letter.

Two films have been based on his books: Auto Focus and Zodiac. Graysmith is portrayed in the film Zodiac by Jake Gyllenhaal.

In early 2010, Graysmith released another book titled: The Girl In Alfred Hitchcock's Shower about the perceived murder of Marli Renfro, who played Janet Leigh's body double in the shower scene of Psycho.

Graysmith was born in Pensacola, Florida as Robert Gray Smith. Before the divorce from his first wife, Margaret Ann, they had two sons together, David and Aaron. After the divorce, he changed his name to Graysmith in 1976.

Graysmith and his second wife, Melanie, an artist at the Chronicle, had a daughter, Margot Alexandra. They separated in 1980 and divorced in 1983.