diff --git a/src/taskgraph/config.py b/src/taskgraph/config.py
index b914b98f0..1f4eca909 100644
--- a/src/taskgraph/config.py
+++ b/src/taskgraph/config.py
@@ -112,6 +112,11 @@ class GraphConfig:
 
     _PATH_MODIFIED = False
 
+    def __post_init__(self):
+        # ensure we have an absolute path; this is required for assumptions
+        # made later, such as the `vcs_root` being a directory above `root_dir`
+        object.__setattr__(self, "root_dir", os.path.abspath(self.root_dir))
+
     def __getitem__(self, name):
         return self._config[name]
 
diff --git a/test/test_config.py b/test/test_config.py
index 596036f34..84e459f85 100644
--- a/test/test_config.py
+++ b/test/test_config.py
@@ -2,6 +2,8 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+import os.path
+
 import pytest
 
 from taskgraph.config import GraphConfig
@@ -10,6 +12,8 @@
 def test_graph_config_basic():
     graph_config = GraphConfig({"foo": "bar"}, "root")
 
+    assert os.path.isabs(graph_config.root_dir)
+
     assert "foo" in graph_config
     assert graph_config["foo"] == "bar"
     assert graph_config.get("foo") == "bar"