diff --git a/pandas/core/sorting.py b/pandas/core/sorting.py
index 1b1d9d7640058..a431842218b3b 100644
--- a/pandas/core/sorting.py
+++ b/pandas/core/sorting.py
@@ -98,8 +98,8 @@ def get_indexer_indexer(
             sort_remaining=sort_remaining,
             na_position=na_position,
         )
-    elif (ascending and target.is_monotonic_increasing) or (
-        not ascending and target.is_monotonic_decreasing
+    elif (np.all(ascending) and target.is_monotonic_increasing) or (
+        not np.any(ascending) and target.is_monotonic_decreasing
     ):
         # Check monotonic-ness before sort an index (GH 11080)
         return None
diff --git a/pandas/tests/series/methods/test_sort_index.py b/pandas/tests/series/methods/test_sort_index.py
index c5d4694f8bdbd..d6817aa179b7b 100644
--- a/pandas/tests/series/methods/test_sort_index.py
+++ b/pandas/tests/series/methods/test_sort_index.py
@@ -317,3 +317,21 @@ def test_sort_values_key_type(self):
         result = s.sort_index(key=lambda x: x.month_name())
         expected = s.iloc[[2, 1, 0]]
         tm.assert_series_equal(result, expected)
+
+    @pytest.mark.parametrize(
+        "ascending",
+        [
+            [True, False],
+            [False, True],
+        ],
+    )
+    def test_sort_index_multi_already_monotonic(self, ascending):
+        # GH 56049
+        mi = MultiIndex.from_product([[1, 2], [3, 4]])
+        ser = Series(range(len(mi)), index=mi)
+        result = ser.sort_index(ascending=ascending)
+        if ascending == [True, False]:
+            expected = ser.take([1, 0, 3, 2])
+        elif ascending == [False, True]:
+            expected = ser.take([2, 3, 0, 1])
+        tm.assert_series_equal(result, expected)