Before 2016, the last time that a Democratic presidential nominee won the San Joaquin Valley was in the Lyndon Johnson landslide of 1964, when President Johnson defeated Republican Senator Barry Goldwater by nearly 140,000 votes.
The closest that Democratic presidential nominees came to winning the San Joaquin Valley counties between 1964 and 2016 was in 1976, followed by 1968. Democrat Hubert Humphrey lost the San Joaquin Valley by 5,657 votes to Republican Richard Nixon in 1968. Democratic Governor Jimmy Carter lost the San Joaquin Valley to Republican President Gerald Ford in 1976 by a similar margin, 5,003 votes.
In 1984, Republican President Ronald Reagan won the eight counties of the San Joaquin Valley by nearly 152,000 votes over Democrat Walter Mondale.
In 2004, Republican President George W. Bush lost California, but won the San Joaquin Valley counties by a 214,000-vote margin over Democrat John Kerry.
In 2012, Republican "Mitt" Romney won the eight San Joaquin Valley counties by 21,475 votes over Democratic President Barack Obama (Romney: 535,698 vs. Obama: 514,223).
The eight counties include some population outside of the San Joaquin Valley floor. For example, Kern County extends across the Tehachapi Mountains into the Mojave Desert and includes a few cities that are outside of the Central Valley, such as Tehachapi, California City and Ridgecrest. These non-Valley cities tend to be much more Republican than the eight-county San Joaquin Valley region as a whole.
Kern County was the state's Republican vote "stronghold" in the 2016 presidential election. No California county had a larger vote margin for Trump, 30,895 ballots. (As discussed in a previous "CalPolitiCal" post, Orange County historically had been California's Republican stronghold, but Orange County voted Democratic by nearly 102,000 votes in 2016.) Clinton's margin in San Joaquin County, 32,188, neutralized Trump's margin in Kern County.
The 2010 U.S. Census of Population found 3.97 million persons in the eight counties of the San Joaquin Valley. If the region were its own state, it would have ranked 27th in the United States by population in 2010, behind Kentucky and just ahead of Oregon. The San Joaquin Valley has a larger population than many prominent agricultural states, such as Iowa, Kansas and Nebraska.
"CalPolitical" examined the San Joaquin Valley in presidential election history in a January 2012 post: http://calpolitical.blogspot.com/2012/01/california-state-route-99-road-to.html