California during World War II was a major contributor to the World War II effort. California's long Pacific Ocean coastline provided the support needed for the Pacific War. California also supported the war in Europe. After the Japanese attack on Pearl Harbor on December 7, 1941, most of California's manufacturing was shifted to the war effort. California became a major ship builder and aircraft manufacturer. Existing military installations were enlarged and many new ones were built. California trained many of the troops before their oversea deployment. Over 800,000 Californians served in the United States Armed Forces. California agriculture, ranches and farms were used to feed the troops around the world. California's long coastline also put the state in fear, as an attack on California seemed likely. California was used for the temporary and permanent internment camps for Japanese Americans. The population grew significantly, largely due to servicemen who were stationed at the new military bases/training facilities and the mass influx of workers from around the U.S. in the growing defense industries. With all the new economy activity, California was lifted out of the Great Depression. Over 500,000 people moved to California from other states to work in the growing economy. California expanded its oil and mineral production to keep up with the war demand.

A total of 12% of all U.S. Government war contracts were awarded to California companies. A total of 17% of the war materials were made in California. Mining, natural gas, and oil production were active industries in California before World War II, and these rapidly expanded to support the war effort. Like World War I, the mines and mining towns came to life again, due to an increase in demand for gold, copper, and silver. California oil production doubled, the synthetic rubber industry created in California and California agricultural output almost tripled. In 1941 California oil production was 230,263,000 barrels; by the end of the war in 1945 the output was 326,555,000 barrels. Raw material was also shipped to California from Lend lease U.S. Allies. After the attack on Pearl Harbor and America entered the war, there was a quick build of new military bases, airfields, training camps, and other military installations. New military construction projects and the emerging war industries in California brought in tens of thousands of workers from across America. After the war, many stayed in California, with some others returning to their home states. Towns and cities next to military and industrial facilities grew and had an increase in the economy. California's population in 1940 was 6,907,387 and by 1950 it had grown to 10,586,223, a 53.3% increase. California received one eighth of all war orders. With the manpower shortage many women entered the workforce in manufacturing and other jobs held by men in the past. As factories added more shifts, a variety of stores and services increase operating hours. To retain workers, some businesses increased their employee benefits. Many military personnel who were trained in California returned after the war to tour the state, so California's tourist industries began to grow.

California's mild climate made it ideal for year-round food production. With many men overseas, there was labor shortage at harvest time. The need for extra workers at harvest brought in housewives and students. Some businesses even loaned workers to help with harvest and food packing as needed. The Woman's Land Army of America was one of the organizations helping in food production. Even with the increase in food production there was mandatory food rationing. Civilians were encouraged to plant Victory gardens to help with the food shortage. The slogan "grow your own, can your own", was started at start of the war and referred to families growing and canning their own food in victory gardens. With its mild climate most victory gardens were grown almost year-round. Tires and gasoline were also rationed. Rationing of wool fabric was also required during the war. This is one of the causes of the June 1943 Zoot Suit Riots in Los Angeles.

California was a major supplier of ammunition for the war. Riverbank Army Ammunition Plant in Stanislaus County and Benicia Arsenal were two of the largest ammunition makers. In San Bernardino the Western Stove Company built incendiary bombs.

The largest World War II accident in California was the Port Chicago disaster. The Liberty ship SS E. A. Bryan exploded on July 17, 1944, while being loaded with ammunition. About 4,600 tons (4,173 tonnes) of explosives had been loaded in the ship's holds at the time. The explosion killed 320 sailors and civilians and injured 390 others.

Hollywood's motion-picture industry continued to make movies during the war. In addition to entertainment films, Hollywood made training films and films to raise morale. The 1942 film The Arm Behind the Army showed how important home support of the war was.^{[citation needed]}

Bob Hope volunteered with the United Service Organizations (USO) and entertained troops during World War II and for decades later (1941 to 1991). Hope brought many Hollywood stars with him on his USO tours.

Desi Arnaz was stationed at Birmingham General Army Hospital in Van Nuys, California during the war to entertain the troops there. Arnaz had a bad knee and so was transferred to the US Army Medical Corps. Arnaz also coordinated with the stars that visited the hospital.