They are grown in Hawaii in volcanic soil. After harvesting, they are left in the fields for a short while to cure, then bagged and shipped to the Continental United States.
They were first popularized by tourists bringing them back from Hawaii.
Maui Sweet are Granex type onions.
A co-op was formed called the Maui Growers Co-op in 1943 to develop these onions from Granex varieties brought in from Texas.