A mutineer who left Hernan Cortez’ expedition sailed up the west coast of Mexico observed the Gulf of California in 1533 and assumed that the place he called California was a large island. He compared it to a fictional island from a novel and the name stuck. And so did the idea that it was an island. Only a few years later, in 1539, the Gulf of California was thoroughly explored and California was found to not be an island after all. But explorers, geographers, and cartographers didn't get the memo, and maps were drawn showing California as an enormous island that went up the entire west coast of what is now the US for the next 300 years! Granted, the last of these maps, dated to 1865, was Japanese and relied on hopelessly outdated information. Still, it is odd that European mapmakers continued to draw California as an island up through the middle of the 18th century.
There were reasons for the misunderstanding, including slow communication between continents and translation between languages, but it was also politics. Spain and England were in a race to claim North America, and cutting off California, claimed by Sir Francis Drake, would leave the mainland open to the Spanish. Read about the island of California at Atlas Obscura. The post has 18 maps made between the 16th and 19th centuries, 17 of which show California as an island.