The most widely shared start and end dates of WWII are 1 September 1939 to 2 September 1945, and between these dates, the first American citizens killed were the 28 civilians killed when SS Athenia was sunk on 3 September 1939.
The full list of those lost is available at
http://www.mikekemble.com/ww2/athenia.html. Where all other sources give the number of US dead as 28, this list (with names) list 30 American citizens, but also 7 Polish and ... 4 Germans !!!