In the one hand, how would I know, not having been brought up in America, but it is hard to believe that America, The Melting Pot, did not have boxes on its forms for mixed race or other.
I am happy to inform you that mixed race was not an option on US census forms until the census of 2000, when Meghan Markle was nineteen.
Sixteen states outlawed interracial marriage until 1967. You can look up what miscegenation means and how much effort went into preventing it, if you wish.
The notion of 'one drop' meant that a single black ancestor in your lineage meant you were black. What do you all not understand about the ramifications of this rule? It's been mentioned several times on this thread.
It is hard to believe that there are people who know so little about American history posting with such confidence about America, and casting doubts on Meghan Markle's lived experience left, right and centre.
FYI, @ancientgran , the 2001 census was the first one in the England and Wales in which mixed race options were were offered. Three specific options plus a write in option were the offerings. Scotland and NI that year only included 'Any Mixed' and 'Mixed' respectively.