diff --git a/swh/loader/mercurial/identify.py b/swh/loader/mercurial/identify.py
--- a/swh/loader/mercurial/identify.py
+++ b/swh/loader/mercurial/identify.py
@@ -3,11 +3,12 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+from codecs import escape_decode  # type: ignore
 import json
+import os
+from pathlib import Path
 import re
 import subprocess
-from codecs import escape_decode  # type: ignore
-from pathlib import Path
 from typing import Any, Dict, Iterator, List, NamedTuple, Optional, Union
 
 # WARNING: do not import unnecessary things here to keep cli startup time under
@@ -196,7 +197,9 @@
 
     def _output(self, *args) -> bytes:
         """Return the outpout of a `hg` call."""
-        return subprocess.check_output(["hg", *args], cwd=self._root)
+        return subprocess.check_output(
+            ["hg", *args], cwd=self._root, env=self._get_env()
+        )
 
     def _call(self, *args) -> None:
         """Perform a `hg` call."""
@@ -205,8 +208,18 @@
             cwd=self._root,
             stderr=subprocess.PIPE,
             stdout=subprocess.PIPE,
+            env=self._get_env(),
         )
 
+    def _get_env(self) -> Dict[str, str]:
+        """Return the smallest viable environment for `hg` suprocesses"""
+        env = {
+            "PATH": os.environ["PATH"],
+            "HGPLAIN": "",  # Tells Mercurial to disable output customization
+            "HGRCPATH": "",  # Tells Mercurial to ignore config files
+        }
+        return env
+
     def root(self) -> Path:
         """Return the root of the Mercurial repository."""
         return self._root
diff --git a/swh/loader/mercurial/tests/conftest.py b/swh/loader/mercurial/tests/conftest.py
--- a/swh/loader/mercurial/tests/conftest.py
+++ b/swh/loader/mercurial/tests/conftest.py
@@ -3,6 +3,7 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+import os
 from typing import Any, Dict
 
 import pytest
@@ -37,3 +38,12 @@
         "max_content_size": 104857600,
         "temp_directory": str(tmp_path),
     }
+
+
+@pytest.fixture(autouse=True, scope="session")
+def swh_mercurial_set_plain():
+    """Mercurial is customizable by users, so we use built-in environment
+    variables to turn off all customization in tests.
+    """
+    os.environ["HGPLAIN"] = ""
+    os.environ["HGRCPATH"] = ""