Kelley died Sunday of complications from osteoporosis in his Petaluma home, according to his publicist, Jennifer Gross.
The artwork that Kelley and his lifelong collaborator, Stanley Mouse, churned out from their studio, a converted firehouse where Janis Joplin first rehearsed with Big Brother and the Holding Company, was iconic. It include dozens of classic rock posters, including the famous Grateful Dead "skull and roses" poster designed for a show at the Avalon Ballroom, as well as posters and album covers for Journey, Steve Miller, Jimi Hendrix and The Beatles.
For inspiration, the pair scrutinized old etchings and photos, took in the youth culture of the time and dug through public libraries, often breaking out into laughter until they were asked to leave by the librarian, Mouse recalled.
"We were just having fun making posters," Mouse told the San Francisco Chronicle. "There was no time to think about what we were doing. It was a furious time, but I think most great art is created in a furious moment."
Born on June 17, 1940, Kelley met Mouse in 1965, at the epicenter of the hippie movement _ San Francisco's Haight-Ashbury district _ and soon recognized their ability to work together, in their words "riffing off each other's giggle."
In recent years, Kelley's artwork focused on paintings of hot rods and custom cars, which were sold as fine art and printed on T-shirts.
He is survived by his wife Marguerite Trousdale Kelley; their children Patty, Yosarian and China; his mother, his sister and two grandchildren.
Memorial plans are pending.