diff --git a/example_usage/lib/library_source.info.dart b/example_usage/lib/library_source.info.dart
index 47ff6ea3..99b7457e 100644
--- a/example_usage/lib/library_source.info.dart
+++ b/example_usage/lib/library_source.info.dart
@@ -1,5 +1,5 @@
-// dart format width=80
 // GENERATED CODE - DO NOT MODIFY BY HAND
+// dart format width=80
 
 // **************************************************************************
 // MemberCountLibraryGenerator
diff --git a/source_gen/CHANGELOG.md b/source_gen/CHANGELOG.md
index 9f6dba4b..44f763b9 100644
--- a/source_gen/CHANGELOG.md
+++ b/source_gen/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 3.1.1-wip
+
+- Keep `// GENERATED FILE` comments on the first line.
+
 ## 3.1.0
 
 - Prepare to stop using `dart:mirrors`: deprecate `TypeChecker.fromRuntime`.
diff --git a/source_gen/lib/src/builder.dart b/source_gen/lib/src/builder.dart
index 6eaba0ff..0892ec42 100644
--- a/source_gen/lib/src/builder.dart
+++ b/source_gen/lib/src/builder.dart
@@ -433,7 +433,14 @@ String _defaultFormatOutput(String code, Version version) =>
 
 /// Prefixes a dart format width and formats [code].
 String _defaultFormatUnit(String code, Version version) {
-  code = '$dartFormatWidth\n$code';
+  if (code.startsWith('$defaultFileHeader\n')) {
+    code =
+        '$defaultFileHeader\n'
+        '$dartFormatWidth\n'
+        '${code.substring(defaultFileHeader.length)}';
+  } else {
+    code = '$dartFormatWidth\n$code';
+  }
   return _defaultFormatOutput(code, version);
 }
 
diff --git a/source_gen/pubspec.yaml b/source_gen/pubspec.yaml
index 546c6ed5..80c7efc9 100644
--- a/source_gen/pubspec.yaml
+++ b/source_gen/pubspec.yaml
@@ -1,5 +1,5 @@
 name: source_gen
-version: 3.1.0
+version: 3.1.1-wip
 description: >-
   Source code generation builders and utilities for the Dart build system
 repository: https://github.com/dart-lang/source_gen/tree/master/source_gen
diff --git a/source_gen/test/builder_test.dart b/source_gen/test/builder_test.dart
index f097d01b..d2dff25f 100644
--- a/source_gen/test/builder_test.dart
+++ b/source_gen/test/builder_test.dart
@@ -1058,8 +1058,8 @@ final int foo = 42
 ''';
 
 const _testGenPartContent = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 part of 'test_lib.dart';
 
@@ -1072,8 +1072,8 @@ part of 'test_lib.dart';
 ''';
 
 const _testGenPartContentForLibrary = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 part of 'test_lib.dart';
 
@@ -1085,8 +1085,8 @@ part of 'test_lib.dart';
 ''';
 
 const _testGenStandaloneContent = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 // **************************************************************************
 // CommentGenerator
@@ -1097,8 +1097,8 @@ $dartFormatWidth
 ''';
 
 const _testGenPartContentForClassesAndLibrary = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 part of 'test_lib.dart';
 
@@ -1112,8 +1112,8 @@ part of 'test_lib.dart';
 ''';
 
 const _testGenNoLibrary = '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 part of 'test_lib.dart';
 
diff --git a/source_gen/test/generator_for_annotation_test.dart b/source_gen/test/generator_for_annotation_test.dart
index 900e547a..88b2220d 100644
--- a/source_gen/test/generator_for_annotation_test.dart
+++ b/source_gen/test/generator_for_annotation_test.dart
@@ -52,8 +52,8 @@ void main() {
       _inputMap,
       outputs: {
         'a|lib/file.g.dart': '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 // **************************************************************************
 // Generator: Repeating
@@ -150,8 +150,8 @@ main() {}''',
       },
       outputs: {
         'a|lib/file.g.dart': '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 // **************************************************************************
 // Generator: Deprecated
@@ -195,8 +195,8 @@ $dartFormatWidth
       },
       outputs: {
         'a|lib/file.g.dart': '''
-$dartFormatWidth
 // GENERATED CODE - DO NOT MODIFY BY HAND
+$dartFormatWidth
 
 // **************************************************************************
 // Generator: Deprecated