Sweaty knickers, I knew I'd get that wrong.
Mold is more Americanised, but the 'U' in our version seems to hark back to some old French influence. So I can't win.
However:
Both spellings have been used since the 16th century. In Canada, both spellings are used.
In New Zealand, "mold" refers to a form for casting a shape while "mould" refers to the fungus