It has been said, and I have not seen anyone dispute it, that most of the Chinese in the U.S. speak Cantonese. During the period in the late 1800s when Chinese emigrated to the West Coast (primarily) and later to other parts of the U.S., most of them came from southern China and spoke Cantonese.

This goes along with what Kaisheng said. If you go to a Chinatown district most of what you hear (if not English) will be Cantonese. Later immigrants have been mostly Mandarin speakers. He also said that he believes that Mandarin has the largest number of native speakers of all the languages we lump together as Chinese.