diff --git a/narwhals/_pandas_like/group_by.py b/narwhals/_pandas_like/group_by.py
index f9ea86dc0..06190bf3e 100644
--- a/narwhals/_pandas_like/group_by.py
+++ b/narwhals/_pandas_like/group_by.py
@@ -11,6 +11,7 @@
 
 from narwhals._expression_parsing import is_simple_aggregation
 from narwhals._expression_parsing import parse_into_exprs
+from narwhals._pandas_like.utils import horizontal_concat
 from narwhals._pandas_like.utils import native_series_from_iterable
 from narwhals._pandas_like.utils import select_columns_by_name
 from narwhals.utils import Implementation
@@ -236,18 +237,11 @@ def agg_pandas(  # noqa: PLR0915
             new_names = [new_names[i] for i in index_map]
             result_simple_aggs.columns = new_names
 
-            # Keep inplace=True to avoid making a redundant copy.
-            # This may need updating, depending on https://github.com/pandas-dev/pandas/pull/51466/files
-            result_simple_aggs.reset_index(inplace=True)  # noqa: PD002
-
         if nunique_aggs:
             result_nunique_aggs = grouped[list(nunique_aggs.values())].nunique(
                 dropna=False
             )
             result_nunique_aggs.columns = list(nunique_aggs.keys())
-            # Keep inplace=True to avoid making a redundant copy.
-            # This may need updating, depending on https://github.com/pandas-dev/pandas/pull/51466/files
-            result_nunique_aggs.reset_index(inplace=True)  # noqa: PD002
         if simple_aggs and nunique_aggs:
             if (
                 set(result_simple_aggs.columns)
@@ -259,7 +253,11 @@ def agg_pandas(  # noqa: PLR0915
                     "that aggregations have unique output names."
                 )
                 raise ValueError(msg)
-            result_aggs = result_simple_aggs.merge(result_nunique_aggs, on=keys)
+            result_aggs = horizontal_concat(
+                [result_simple_aggs, result_nunique_aggs],
+                implementation=implementation,
+                backend_version=backend_version,
+            )
         elif nunique_aggs and not simple_aggs:
             result_aggs = result_nunique_aggs
         elif simple_aggs and not nunique_aggs:
@@ -269,6 +267,9 @@ def agg_pandas(  # noqa: PLR0915
             result_aggs = native_namespace.DataFrame(
                 list(grouped.groups.keys()), columns=keys
             )
+        # Keep inplace=True to avoid making a redundant copy.
+        # This may need updating, depending on https://github.com/pandas-dev/pandas/pull/51466/files
+        result_aggs.reset_index(inplace=True)  # noqa: PD002
         return from_dataframe(
             select_columns_by_name(
                 result_aggs, output_names, backend_version, implementation