Well they're both big continents so mileage could range greatly. But if you mean shortest distance it's about 3,886 miles which is about from South Carolina to Mauritania. But if I calculated the center of the US to the center of Africa it's about 7,566 miles.
Bangor, Maine, to Agadir, Morocco, is only 3332 miles on a great circle. That's 500 miles closer than South Carolina to Mauritania. I suspect there are still shorter routes.