diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index 8bb8f00b4c406..7af32d70c00bc 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -431,16 +431,9 @@ def _wrap_applied_output(
             )
         assert values is not None
 
-        def _get_index() -> Index:
-            if self.grouper.nkeys > 1:
-                index = MultiIndex.from_tuples(keys, names=self.grouper.names)
-            else:
-                index = Index._with_infer(keys, name=self.grouper.names[0])
-            return index
-
         if isinstance(values[0], dict):
             # GH #823 #24880
-            index = _get_index()
+            index = self._group_keys_index
             res_df = self.obj._constructor_expanddim(values, index=index)
             res_df = self._reindex_output(res_df)
             # if self.observed is False,
@@ -453,7 +446,7 @@ def _get_index() -> Index:
         else:
             # GH #6265 #24880
             result = self.obj._constructor(
-                data=values, index=_get_index(), name=self.obj.name
+                data=values, index=self._group_keys_index, name=self.obj.name
             )
             return self._reindex_output(result)
 
diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
index a60ec29581337..d83b3ee1738bf 100644
--- a/pandas/core/groupby/groupby.py
+++ b/pandas/core/groupby/groupby.py
@@ -1177,6 +1177,18 @@ def _resolve_numeric_only(self, numeric_only: bool | lib.NoDefault) -> bool:
         # expected "bool")
         return numeric_only  # type: ignore[return-value]
 
+    @cache_readonly
+    def _group_keys_index(self) -> Index:
+        # The index to use for the result of Groupby Aggregations.
+        # This _may_ be redundant with self.grouper.result_index, but that
+        #  has not been conclusively proven yet.
+        keys = self.grouper._get_group_keys()
+        if self.grouper.nkeys > 1:
+            index = MultiIndex.from_tuples(keys, names=self.grouper.names)
+        else:
+            index = Index._with_infer(keys, name=self.grouper.names[0])
+        return index
+
     # -----------------------------------------------------------------
     # numba
 
@@ -1244,7 +1256,7 @@ def _aggregate_with_numba(self, data, func, *args, engine_kwargs=None, **kwargs)
         data and indices into a Numba jitted function.
         """
         starts, ends, sorted_index, sorted_data = self._numba_prep(func, data)
-        group_keys = self.grouper._get_group_keys()
+        index = self._group_keys_index
 
         numba_agg_func = numba_.generate_numba_agg_func(kwargs, func, engine_kwargs)
         result = numba_agg_func(
@@ -1252,7 +1264,7 @@ def _aggregate_with_numba(self, data, func, *args, engine_kwargs=None, **kwargs)
             sorted_index,
             starts,
             ends,
-            len(group_keys),
+            len(index),
             len(data.columns),
             *args,
         )
@@ -1261,10 +1273,6 @@ def _aggregate_with_numba(self, data, func, *args, engine_kwargs=None, **kwargs)
         if cache_key not in NUMBA_FUNC_CACHE:
             NUMBA_FUNC_CACHE[cache_key] = numba_agg_func
 
-        if self.grouper.nkeys > 1:
-            index = MultiIndex.from_tuples(group_keys, names=self.grouper.names)
-        else:
-            index = Index(group_keys, name=self.grouper.names[0])
         return result, index
 
     # -----------------------------------------------------------------