l2 is not a sublist of l1.

But in general, sublist checking is quite easy to do.

def sublist?([], _), do: false
def sublist?(l1 = [_|t], l2) do
  List.starts_with?(l1, l2) or sublist?(t, l2)
end

def subset(_, []), do: true
def subset([], _), do: false
def subset([h | t1], [h | t2]), do: subset(t1, t2)
def subset([_ | t], list), do: subset(t, list)

According to @NobbZ 's suggestion as well, I tried this one works:

big_list = ["9", "2", "5", "1"]
sub_out_of_order_list = ["1", "9"]
MapSet.subset?(MapSet.new(sub_out_of_order_list), MapSet.new(big_list))

Thank you everyone.

Depending on the relative sizes of your lists, this could also be a viable option:

Enum.all?(sub_out_of_order_list, &Enum.member?(big_list, &1))

Maybe I need to clarify a bit, actually I don’t mean sublist, I mean a list’s all items in another list, don’t care order.