diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index b68b029ecf..e7d77f22e4 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   The original string assigned to a model attribute is no longer incorrectly
+    frozen.
+
+    Fixes #24185, #28718.
+
+    *Matthew Draper*
+
 *   Avoid converting integer as a string into float.
 
     *namusyaka*
diff --git a/activemodel/lib/active_model/type/string.rb b/activemodel/lib/active_model/type/string.rb
index 8a91410998..850cab962b 100644
--- a/activemodel/lib/active_model/type/string.rb
+++ b/activemodel/lib/active_model/type/string.rb
@@ -11,9 +11,14 @@ module ActiveModel
 
       private
 
-      def cast_value(value)
-        ::String.new(super)
-      end
+        def cast_value(value)
+          case value
+          when ::String then ::String.new(value)
+          when true then "t".freeze
+          when false then "f".freeze
+          else value.to_s
+          end
+        end
     end
   end
 end
diff --git a/activemodel/test/cases/type/string_test.rb b/activemodel/test/cases/type/string_test.rb
index 7b25a1ef74..5d024089d0 100644
--- a/activemodel/test/cases/type/string_test.rb
+++ b/activemodel/test/cases/type/string_test.rb
@@ -10,6 +10,18 @@ module ActiveModel
       assert_equal "123", type.cast(123)
     end
 
+    test "cast strings are mutable" do
+      type = Type::String.new
+
+      s = "foo"
+      assert_equal false, type.cast(s).frozen?
+      assert_equal false, s.frozen?
+
+      f = "foo".freeze
+      assert_equal false, type.cast(f).frozen?
+      assert_equal true, f.frozen?
+    end
+
     test "immutable strings are not duped coming out" do
       s = "foo"
       type = Type::ImmutableString.new
@@ -18,10 +30,13 @@ module ActiveModel
     end
 
     test "values are duped coming out" do
-      s = "foo"
       type = Type::String.new
+
+      s = "foo"
       assert_not_same s, type.cast(s)
+      assert_equal s, type.cast(s)
       assert_not_same s, type.deserialize(s)
+      assert_equal s, type.deserialize(s)
     end
   end
 end