1. At the age of eleven Diehl moved with his family from Pittsburgh to Pleasant Hill, California In 1968 he started taking courses in the art department at Diablo Valley College and then studied under Mel Ramos, one of the originators of figurative Pop Art at Cal State University, Hayward, earning a BA in 1973.
Guy Diehl says that Ramos taught him "discipline and professionalism" and, crucially, to incorporate the camera as a tool in his painting practice.
Ramos encouraged Guy Diehl to attend San Francisco State University for graduate work.
Guy Diehl continued painting in a style heavily influenced by his photorealist mentors for several years: a 1980 review in Artweek notes a reverence for Wayne Thiebaud in both the title of Guy Diehl's 1980 watercolor Thiebaud Towel and the execution of his acrylic painting Green Deck Chair from the same year.
Los Angeles Times art critic Leah Ollman writes that Guy Diehl "practices painting as an act of homage," citing "visual quotes" and references to artists including Morandi, Rothko, Zurbaran, Ingres, Goya, Modigliani, Demuth and Joan Brown in a 2004 review of Guy Diehl's paintings at Schlesinger Gallery.
Guy Diehl's work has generally taken a serial form, with paintings or prints issued in a series connected by some common theme, from his 1970s images of figures in swimming pools to his more recent series of book paintings.
Since 1988, Guy Diehl has collaborated with Donald Farnsworth at Magnolia Editions in Oakland, California on a variety of print projects including lithographs, etchings, woodcuts, pigmented inkjet prints and mixed-media editions.
Guy Diehl is currently represented by Dolby Chadwick Gallery in San Francisco; he was previously represented by Hackett-Freedman Gallery in San Francisco.
Guy Diehl has had solo exhibitions at San Francisco venues including Modernism ; Jeremy Stone Gallery ; and Hank Baum Gallery, as well as exhibiting in Stockton, Redding, Pittsburgh, and Concord, California, and at the Shepard Art Gallery in Reno, Nevada.
Guy Diehl has been commissioned to create works for the Mansion at MGM Grand; The One and Only Ocean Club, Paradise Island, Bahamas; the Peninsula Hotel in New York City; Princess Cruise Lines; Vasco Restaurant in Mill Valley, California; and Voltaire Restaurant in Dallas, Texas.
In 2013, Guy Diehl created a nine-by-eleven-foot mural entitled Still Life with Billie Holiday, a "transmedia" work with embedded RFID tags, at 240 G Street in downtown Davis, California as part of an interactive art walk installation organized by John Natsoulas Gallery.