"Rivera was like Paul Revere, he's yelling 'the Nazis are coming, the Nazis are coming,'" said Will Maynez who has studied one of Rivera's most recognized murals called "Pan American Unity" which highlights World War II.
"Once Hitler won in Europe and everybody thought he was, he was going to turn his malignant eye to the Americas. We had to be a unified front," he explained.
Maynez told ABC7 that Rivera envisioned unity between Mexico's rich culture and the industrialized United States. That's the theme behind the mural.
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In 1940 Rivera began painting on ten steel-framed panels made of wire mesh and cement with limestone plaster as the base.
This way it could be transported to City College of San Francisco where it was stored and eventually moved to an impractical location, crammed inside the small performing arts theater.
What for years people have admired is really only one-third of Rivera's concept.
"Everything got put on hold because of World War II and then after World War II it nicely segued into the Cold War, Rivera is a commie, he never comes back," revealed Maynez.
City College plans to eventually display it in a much larger, to-be-built, public space.
In the meantime, the mural is on loan to the San Francisco Museum of Modern Art for at least two years, free to the public.
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Getting it to the museum took a team of experts and engineers from Mexico to safely stabilize and put each panel in custom crates and transport them across the city.
Maynez loves the space it now has at the MOMA. "You can get up there, you can really interact with the mural so I'm, I'm just 'encantado.'" He says, he's delighted.
In 1926, in another part of town, the San Francisco Art Institute had a new building and plenty of empty space.
"This is a perfect opportunity to have Diego Rivera come and do a mural in California," said Jeff Gunderson, the institute's librarian and archivist.
Four years later, in 1930, then-president of the board of trustees commissioned his friend Rivera to paint what is now known as "The Making of a Fresco." It took Rivera one month to complete.
He doesn't forget to acknowledge his financial supporters but clearly the central figure is a blue collar worker.
"He puts a red star, a red badge in his pocket there to symbolize his good left wing, commie leanings that Rivera had at that time," explained Gunderson.
Rivera painted himself, shown seated in the middle as the mural is being worked on.
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Today, you would be surprised at what Rivera charged them. The signed archived receipts show one payment for $2,500, the other for $500.
"Which was a lot of money at that time," insisted Gunderson.
That same year Rivera's San Francisco connections brought him to the exclusive City Club of San Francisco. This time, "The Allegory of California" shows a woman representing the state of California, supporting its people and its innovations at the time.
Again, Gunderson of the art institute says Rivera highlights the average workers that keep the state afloat.
"When you look at it, you see the lumber industry and you see the mining industry and it looks like an environmental disaster was happening but he honors all that as labor and how people are going to us natural resources to better themselves," he added.
The last Rivera mural in the Bay Area is located at UC Berkeley. It was painted at the home of Rosalie Meyer Stern in Atherton. When the university named a dorm after Stern, the mural eventually followed in 1956. The small mural, called "Still Life and Blossoming Almond Trees," has a plexiglass barrier.
The children in the mural, descendants of Levi Strauss, were Rosalie Stern's grandchildren. The mural was a gift to the family who hosted him and his wife Frida Kahlo.
"It shows both the elite family, although they are actually kicking back with the overalls but they are enjoying the fruits of these men's labor," said Gray Brechin, a UC Berkeley scholar.
As always, the Mexican farmworker has an honorable place in society because of Rivera's words, "The best I have done, grew out of things deeply felt."