diff --git a/src/config/source/ArcanistFilesystemConfigurationSource.php b/src/config/source/ArcanistFilesystemConfigurationSource.php
index ffcb5016..4281c830 100644
--- a/src/config/source/ArcanistFilesystemConfigurationSource.php
+++ b/src/config/source/ArcanistFilesystemConfigurationSource.php
@@ -1,45 +1,58 @@
 <?php
 
 abstract class ArcanistFilesystemConfigurationSource
   extends ArcanistDictionaryConfigurationSource {
 
   private $path;
 
   public function __construct($path) {
     $this->path = $path;
 
     $values = array();
     if (Filesystem::pathExists($path)) {
       $contents = Filesystem::readFile($path);
       if (strlen(trim($contents))) {
         $values = phutil_json_decode($contents);
       }
     }
 
     $values = $this->didReadFilesystemValues($values);
 
     parent::__construct($values);
   }
 
   public function getPath() {
     return $this->path;
   }
 
   public function getSourceDisplayName() {
     return pht('%s (%s)', $this->getFileKindDisplayName(), $this->getPath());
   }
 
   abstract public function getFileKindDisplayName();
 
   protected function didReadFilesystemValues(array $values) {
     return $values;
   }
 
   protected function writeToStorage($values) {
     $content = id(new PhutilJSON())
       ->encodeFormatted($values);
 
-    Filesystem::writeFile($this->path, $content);
+    $path = $this->path;
+
+    // If the containing directory does not exist yet, create it.
+    //
+    // This is expected when (for example) you first write to project
+    // configuration in a Git working copy: the ".git/arc" directory will
+    // not exist yet.
+
+    $dir = dirname($path);
+    if (!Filesystem::pathExists($dir)) {
+      Filesystem::createDirectory($dir, 0755, $recursive = true);
+    }
+
+    Filesystem::writeFile($path, $content);
   }
 
 }