The sixth and final mural, whose style might be best described as Expressionism meets Saturday morning cartoons, depicts rodeos, fairs, football games and other public events. It fairly explodes with comical, hatchet-faced characters, from flamenco dancers at a mission fiesta to fans thronging a Hollywood premiere. “That mural is 100 percent original,” Mr. Santos said. “I wanted to give the guys something to laugh at, but I can tell you that I worked real fast on it, since my parole date was coming up. I didn’t want to give anyone an excuse to keep me.”

Eduardo Pineda, a longtime community muralist, is the director of education for the Museum of the African Diaspora in nearby San Francisco and has visited the murals. “What Santos does with space and perspective is very sophisticated,” he said. “The murals are highly cinematic, with powerful narration.”

Though for decades the murals remained in excellent condition, they now are defaced in places by prisoner graffiti. “San Quentin used to be populated exclusively by long-term inmates, and respect for the murals was part of the older prison culture,” said Lt. Eric Messick, San Quentin’s public information officer. “But around 1988 we also became a reception center for newly sentenced inmates, and most of them are younger guys who just don’t have the connection with the murals. That’s when the tagging started.”

Steve Emerick, who runs the prison’s Arts in Corrections program, said that the inmates themselves could do the required restoration if they were trained or supervised by an expert. “But that requires funding we simply don’t have,” he said. Restorations were carried out in the late 1960s, when a clear protective coating was applied, and again in the 1990s.

A greater threat to the murals may be the continuing tug of war over closing all or part of San Quentin. Established in 1852, it is California’s oldest and possibly most dilapidated prison, and its 432 prime waterfront acres are coveted by developers and local governments. A 2003 preliminary redevelopment study called for preserving the murals but without indicating how or where. A current bill in the California Assembly would prohibit construction of a proposed new death row at San Quentin until the state studies moving the execution chamber elsewhere.

After his parole in 1955, Mr. Santos worked at Disneyland as a caricaturist and then opened a studio and gallery in San Diego, his hometown. But after pleading guilty to possession of marijuana, he fled to Mexico, where he owned a succession of galleries in Guadalajara, Mexico City and Acapulco. Returning to the United States in 1967, he painted, made sculptures of carved wood and found objects and ran a popular gallery and bohemian gathering spot in the Catskills village of Fleischmanns, N.Y. (An exhibition of his work is on display there through Aug. 31 at the Art et cetera gallery.) More than 20 years ago, after a divorce and a heart attack, he moved back to San Diego.