The origin of the California roll is somewhat murky, but usually food historians credit Ichiro Mashita, sushi chef at the Tokyo Kaikan in Los Angeles with inventing the roll in the early 1970s. Mashita realized the oily texture of avocado was a perfect substitute for toro. He also eventually made the roll “inside-out”, i.e. uramaki, because Americans did not like seeing and chewing the nori on the outside of the roll.

Links:
The history of the California Roll - Wikipedia