OK. I'm not really that much into tracing family trees and such but you might find this interesting.
Recently I posted about Maurice Roche, 4th Baron Fermoy who's wife was Lady Ruth Fermoy, a good friend of the Queen Mum. Going back one step furthur, we have American socialite Frances Work who married James, 3rd Baron Fermoy who's oldest son was Maurice. So that makes her Diana's great grandmother.
Another interesting tidbit is that Harry and William, having Frances Work Roche as a great great grandmother, through this lineage they also have Oliver Platt, an American actor who shares the same great great grandmother.
https://en.wikipedia.org/wiki/Frances_Ellen_Work
https://famouskin.com/famous-kin-ch...cess+diana&via=6317+james+boothby+burke+roche
https://en.wikipedia.org/wiki/Oliver_Platt
So, this kind of shows that should Harry and Meghan Markle get engaged and married, he wouldn't be the first in his family tree to have married an American.