Growing up, my idea of "Chinese" was Cantonese. As an ethnic Chinese born and raised in the States, I primarily encountered English on a regular basis outside and Cantonese at home; I rarely encountered Mandarin when I was young, or even when I did, I didn't know it was also "Chinese." My parents and all of their Chinese friends primarily spoke Cantonese, I attended a Chinese school that held instruction in Cantonese, and frequented a Chinatown in which most people I saw as Chinese spoke Cantonese.
Fascinating!