When and how did sushi become popular in the US?
It started in the late 60s in LA, but it's through the late 70s/early 80s that it became really popular with the trend-setting movie people. By then the chefs had already started to move away from traditional Japanese sushi to add a fusion element to their dishes but by the end of the 80s the sushi rolls had totally become the stars, and "American sushi" was truly born.
In the 80s there was a first wave of sushi bars opening in the cities on the continents, still very often Japanese owned, serving sushi at the counter, with a few tables. By the early 90s, bigger places, with many chefs, able to serve tons of maki platters at tables had become the standard.
As the popularity of sushi spread, its quality generally declined, along with the prices. Tuna, salmon and tilapia (often sold under a false appellation like Tai) became the norm. It's only with the appearance of "mass market" sushi, centred on fat or sugar heavy ingredients and "big flavours" (avocado, tempura and flakes, mayonnaise, sriracha, very sweet sauces etc.) in rolled sushi that sushi truly became a very popular food with the masses, around the early to mid 90s I would say.