The last year in which no US citizen was awarded a Nobel prize in economics: 1999. And that was a Canadian.
Note also that this is true even if you are only counting "natural born" US citizens, so this isn't an artifact of the dominance of US universities in economics.
How likely is this sort of trend to be in, say, 20 years? Conventional wisdom says it is likely to end with the larger number of researchers from other countries, but a mental check of the 40-50 year olds at the top of the profession is still dominated by Americans.