JLightnin' wrote that Dave Sime (2nd in 1960 Olympic 100m) "may have been the last caucasian sprinter to qualify for the US in the 100 meter dash)."
Interestingly, just four years earlier in the 1956 Olympics at Melbourne, USA caucasians Bobby Morrow and Thane Baker captured four of the six OG sprint medals. They ran 1-2 in the 100m (and 1-3 in the 200m where Morrow won in OG record time).
Earlier in 1956 Jim Lea of USA set the 440-yard world record, suggesting a third caucasian dash winner at Melbourne - in the 400m. But that did not come to pass because Lea, then in the U.S. military, was subsequently injured.