Also depends on what you want to call American.
Is it citizenship? Birthplace? Ethnicity? Some kind of lineage?
What if the person grew up in two difference places? Parents from different backgrounds?
What if they were born in once place, but immediately moved and were raised in another country?
(As you might expect, there are variations of all these in US GMs).
Looks like Kamsky was born in Russia, and moved to US when he was 15.
Caruana was born in US, then at 12 moved around Europe (Portugal, Italy, Switzerland) to train and play (Europe has the best tournaments and coaches, so this is a common enough sort of story).
Since Caruana just won the Candidates tournament earlier, I've heard that he is the first contender since Kamsky in the 90s, and I've heard he is the first since Fischer. Who was our last representative for the title match?