American Samoa, Guam, Baker Island, Howland Island, and the southern most island of the Northern Mariana Islands - Rota.
All of these islands are closer to the continent of Australia than to mainland Asia or either America.
American Samoa is even down under - below the equator.
Hawaii is a good guess but itself closer to the continent of North America.
And remember! America even has a grand population of marsupials after all - the opossums. They come in over 60 flavors (species).
Finally Jarvis Island is down under - below the equator - and geographically closest to the continent of North America.
What America doesn't have are the egg laying mammals like the platypus and echidna. But we do parthenogenic (no males needed for reproduction) lizards! Something of science fiction that is not fiction - remember Enemy Mine 1985?
I guess parthenogenic lizards are distantly related to the mythical Dragon?