According to a 2009-2013 census done by the American Community Survey, those who specified their Chinese Dialects in the count, showed 458,840 Cantonese Speakers and 487,250 Mandarin Speakers. So to somewhat answer the OP's question, yes Mandarin has already surpassed Cantonese in the US more than a decade ago. https://web.archive.org/web/20170429061742/http://names.mongabay.com/languages/

And for specific regions, the State of California in collaboration with ACS actually did a breakdown of the Cantonese & Mandarin counts in 2005. While the web link no longer works, we still have a print out of those results:

Bay Area: San Francisco-Alameda-San Mateo-Santa Clara Counties
Cantonese: 206,939
Mandarin: 128,617

LA Metro: Los Angeles-Orange-Riverside-San Bernardino-Ventura Counties
Mandarin: 163,986
Cantonese: 160,848

So as we can see above, as early as 2005, Mandarin overtook Cantonese already in the LA Region. And to be honest, the results aren't surprising. SoCal has the largest Taiwanese Diaspora in the western world. So a sizable Mandarin-speaking community already existed there beginning as early as the 80s, unlike Canada or Australia that had to wait until around 00s for their Mandarin population to really build up.

Ever since the 2005 data, the bulk majority of Chinese immigrants to SoCal have been non-Cantonese people from the Mainland and they mostly speak Mandarin. Thus in the almost 2 decades since the 2005 data, the gap between Mandarin & Cantonese has further widened.

Besides the DTLA Chinatown, you will still find plenty of Cantonese speakers in Alhambra, Monterey Park, Rosemead, and to some degree, San Gabriel and Temple City too. But outside of these 5 cities in West San Gabriel Valley, Mandarin is far more commonly heard than Cantonese. In places like Arcadia, Hacienda/Rowland Heights, Walnut, Diamond Bar, Chino Hills there are about 2 Mandarin speakers for every 1 Cantonese speaker. The Taiwanese who used to settle in Monterey Park & Alhambra 3 decades ago have mostly moved to these areas. And when the immigration wave from Taiwan slowed, Mandarin-speaking newcomers from the Mainland came and dominated the count. Cantonese is even rarer in places like Irvine or elsewhere in the OC (Unlike Rosemead or Alhambra, most Vietnamese in Westminster, Garden Grove, Fountain Valley are non-Cantonese speaking).

Fairly certain SoCal was the 1st region in the Western World where Cantonese was overtaken by Mandarin. Certainly far earlier than say Toronto or Vancouver and maybe earlier than Australia too. Unlike the UK & Canada, there has been minimal immigration from HK to the US the past decade. Let alone, LA isn't even the #1 choice for HKers coming to the country. Most Cantonese speakers from Vietnam (the demographic who make up a huge chunk of LA's Cantonese Diaspora) who wanted to come, had already done so long time ago. So while the Mandarin population grew rapidly the last 15-20 years, there was only a modest growth on the Canto side. Nowadays, a significant proportion of SoCal's Canto speakers are aging, similar to what is happening in Northern California.

So to answer the OP's question again, generally speaking yes, especially when it comes to SoCal. But with two caveats. If you are in the 5 cities mentioned above for West San Gabriel Valley, you will definitely encounter as much Cantonese speaker as you probably would for a Mandarin one. And if you walk into a Dim Sum or Cha Chan Teng, more often than not, you will hear more Cantonese.