When did the TV series Kung Fu with David Carradine start broadcasting?
FWIW I never heard of ninjas until Shogun which I see was broadcast in 1980. I actually didn’t see most of the miniseries but I read the book about the same time and definitely remember the ninjas.
The book and miniseries were a pretty big deal back then so I wouldn’t be surprised if a lot of sheltered Americans (like I definitely was) were introduced to the ninja legend through that route.