Guerrilla Gravity makes their carbon frames in house now in Colorado, if you're looking for an modern dual suspension bike that would be my first choice if I had to buy USA made.
Honestly I don't care where the bike is made, I mostly buy used anyway so I'm not really supporting anyone aside from the previous owner.
I do usually buy all my parts and supplies from US based online stores, but I work on my own bikes since I don't like the prices or wait time at local bike shops
What type of bike mountain bike are you looking for?