diff --git a/actionpack/lib/action_controller/renderer.rb b/actionpack/lib/action_controller/renderer.rb
index d91c40c054..5a43e3b3f9 100644
--- a/actionpack/lib/action_controller/renderer.rb
+++ b/actionpack/lib/action_controller/renderer.rb
@@ -99,6 +99,7 @@ module ActionController
       instance.set_response! controller.make_response!(request)
       instance.render_to_string(*args)
     end
+    alias_method :render_to_string, :render # :nodoc:
 
     private
       def normalize_keys(defaults, env)
diff --git a/actiontext/app/helpers/action_text/content_helper.rb b/actiontext/app/helpers/action_text/content_helper.rb
index 17b50fc556..0d0bb19de2 100644
--- a/actiontext/app/helpers/action_text/content_helper.rb
+++ b/actiontext/app/helpers/action_text/content_helper.rb
@@ -10,6 +10,7 @@ module ActionText
     mattr_accessor(:scrubber)
 
     def render_action_text_content(content)
+      self.prefix_partial_path_with_controller_namespace = false
       sanitize_action_text_content(render_action_text_attachments(content))
     end
 
diff --git a/actiontext/lib/action_text.rb b/actiontext/lib/action_text.rb
index 0dd6318f89..15fdf70b3b 100644
--- a/actiontext/lib/action_text.rb
+++ b/actiontext/lib/action_text.rb
@@ -16,6 +16,7 @@ module ActionText
   autoload :Fragment
   autoload :HtmlConversion
   autoload :PlainTextConversion
+  autoload :Rendering
   autoload :Serialization
   autoload :TrixAttachment
 
diff --git a/actiontext/lib/action_text/attachments/trix_conversion.rb b/actiontext/lib/action_text/attachments/trix_conversion.rb
index 15319f4e37..2829da1f59 100644
--- a/actiontext/lib/action_text/attachments/trix_conversion.rb
+++ b/actiontext/lib/action_text/attachments/trix_conversion.rb
@@ -28,7 +28,7 @@ module ActionText
       private
         def trix_attachment_content
           if partial_path = attachable.try(:to_trix_content_attachment_partial_path)
-            ActionText::Content.renderer.render(partial: partial_path, object: self, as: model_name.element)
+            ActionText::Content.render(partial: partial_path, object: self, as: model_name.element)
           end
         end
     end
diff --git a/actiontext/lib/action_text/content.rb b/actiontext/lib/action_text/content.rb
index 16bc6fe031..6126a4de49 100644
--- a/actiontext/lib/action_text/content.rb
+++ b/actiontext/lib/action_text/content.rb
@@ -1,12 +1,8 @@
 # frozen_string_literal: true
 
-require "active_support/core_ext/module/attribute_accessors_per_thread"
-
 module ActionText
   class Content
-    include Serialization
-
-    thread_cattr_accessor :renderer
+    include Rendering, Serialization
 
     attr_reader :fragment
 
@@ -88,7 +84,7 @@ module ActionText
     end
 
     def to_rendered_html_with_layout
-      renderer.render(partial: "action_text/content/layout", locals: { content: self })
+      render partial: "action_text/content/layout", locals: { content: self }
     end
 
     def to_s
diff --git a/actiontext/lib/action_text/engine.rb b/actiontext/lib/action_text/engine.rb
index 8749498158..9d5fd1dd25 100644
--- a/actiontext/lib/action_text/engine.rb
+++ b/actiontext/lib/action_text/engine.rb
@@ -37,21 +37,24 @@ module ActionText
     end
 
     initializer "action_text.helper" do
-      ActiveSupport.on_load(:action_controller_base) do
-        helper ActionText::Engine.helpers
+      %i[action_controller_base action_mailer].each do |abstract_controller|
+        ActiveSupport.on_load(abstract_controller) do
+          helper ActionText::Engine.helpers
+        end
       end
     end
 
-    initializer "action_text.renderer" do |app|
-      app.executor.to_run      { ActionText::Content.renderer = ApplicationController.renderer }
-      app.executor.to_complete { ActionText::Content.renderer = ApplicationController.renderer }
-
+    initializer "action_text.renderer" do
       ActiveSupport.on_load(:action_text_content) do
-        self.renderer = ApplicationController.renderer
+        self.renderer = Class.new(ActionController::Base).renderer
       end
 
-      ActiveSupport.on_load(:action_controller_base) do
-        before_action { ActionText::Content.renderer = ApplicationController.renderer.new(request.env) }
+      %i[action_controller_base action_mailer].each do |abstract_controller|
+        ActiveSupport.on_load(abstract_controller) do
+          around_action do |controller, action|
+            ActionText::Content.with_renderer(controller, &action)
+          end
+        end
       end
     end
 
diff --git a/actiontext/lib/action_text/rendering.rb b/actiontext/lib/action_text/rendering.rb
new file mode 100644
index 0000000000..caef71c6b4
--- /dev/null
+++ b/actiontext/lib/action_text/rendering.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+require "active_support/concern"
+require "active_support/core_ext/module/attribute_accessors_per_thread"
+
+module ActionText
+  module Rendering #:nodoc:
+    extend ActiveSupport::Concern
+
+    included do
+      thread_cattr_accessor :renderer, instance_accessor: false
+      delegate :render, to: :class
+    end
+
+    class_methods do
+      def with_renderer(renderer)
+        previous_renderer = self.renderer
+        self.renderer = renderer
+        yield
+      ensure
+        self.renderer = previous_renderer
+      end
+
+      def render(*args, &block)
+        renderer.render_to_string(*args, &block)
+      end
+    end
+  end
+end
diff --git a/actiontext/test/dummy/app/controllers/admin/messages_controller.rb b/actiontext/test/dummy/app/controllers/admin/messages_controller.rb
new file mode 100644
index 0000000000..7c3316324b
--- /dev/null
+++ b/actiontext/test/dummy/app/controllers/admin/messages_controller.rb
@@ -0,0 +1,5 @@
+class Admin::MessagesController < ActionController::Base
+  def show
+    @message = Message.find(params[:id])
+  end
+end
diff --git a/actiontext/test/dummy/app/controllers/application_controller.rb b/actiontext/test/dummy/app/controllers/application_controller.rb
deleted file mode 100644
index 09705d12ab..0000000000
--- a/actiontext/test/dummy/app/controllers/application_controller.rb
+++ /dev/null
@@ -1,2 +0,0 @@
-class ApplicationController < ActionController::Base
-end
diff --git a/actiontext/test/dummy/app/controllers/messages_controller.rb b/actiontext/test/dummy/app/controllers/messages_controller.rb
index ec85acb10d..b323d50a27 100644
--- a/actiontext/test/dummy/app/controllers/messages_controller.rb
+++ b/actiontext/test/dummy/app/controllers/messages_controller.rb
@@ -1,4 +1,4 @@
-class MessagesController < ApplicationController
+class MessagesController < ActionController::Base
   before_action :set_message, only: [:show, :edit, :update, :destroy]
 
   # GET /messages
diff --git a/actiontext/test/dummy/app/mailers/messages_mailer.rb b/actiontext/test/dummy/app/mailers/messages_mailer.rb
new file mode 100644
index 0000000000..ce0af3b3ca
--- /dev/null
+++ b/actiontext/test/dummy/app/mailers/messages_mailer.rb
@@ -0,0 +1,6 @@
+class MessagesMailer < ApplicationMailer
+  def notification
+    @message = params[:message]
+    mail to: params[:recipient], subject: "NEW MESSAGE: #{@message.subject}"
+  end
+end
diff --git a/actiontext/test/dummy/app/views/admin/messages/show.html.erb b/actiontext/test/dummy/app/views/admin/messages/show.html.erb
new file mode 100644
index 0000000000..ba9067f201
--- /dev/null
+++ b/actiontext/test/dummy/app/views/admin/messages/show.html.erb
@@ -0,0 +1,16 @@
+<dl>
+  <dt>Subject</dt>
+  <dd>
+    <span id="subject"><%= @message.subject %></span>
+  </dd>
+
+  <dt>Content (Plain Text)</dt>
+  <dd>
+    <pre id="content-plain"><%= @message.content.to_plain_text %></pre>
+  </dd>
+
+  <dt>Content (HTML)</dt>
+  <dd>
+    <div id="content-html"><%= @message.content %></div>
+  </dd>
+</dl>
diff --git a/actiontext/test/dummy/app/views/layouts/application.html.erb b/actiontext/test/dummy/app/views/layouts/application.html.erb
index f221f512b7..281e4e3063 100644
--- a/actiontext/test/dummy/app/views/layouts/application.html.erb
+++ b/actiontext/test/dummy/app/views/layouts/application.html.erb
@@ -3,9 +3,6 @@
   <head>
     <title>Dummy</title>
     <%= csrf_meta_tags %>
-
-    <%= stylesheet_link_tag 'application', media: 'all' %>
-    <%= javascript_pack_tag 'application' %>
   </head>
 
   <body>
diff --git a/actiontext/test/dummy/app/views/messages/show.html.erb b/actiontext/test/dummy/app/views/messages/show.html.erb
index 25fad1efba..50fa953421 100644
--- a/actiontext/test/dummy/app/views/messages/show.html.erb
+++ b/actiontext/test/dummy/app/views/messages/show.html.erb
@@ -1,13 +1,8 @@
 <p id="notice"><%= notice %></p>
 
-<p>
-  <strong>Subject:</strong>
-  <%= @message.subject %>
-</p>
+<h1 id="subject"><%= @message.subject %></h1>
 
-<div class="trix-content">
-  <%= @message.content.html_safe %>
-</div>
+<div id="content"><%= @message.content %></div>
 
 <%= link_to 'Edit', edit_message_path(@message) %> |
 <%= link_to 'Back', messages_path %>
diff --git a/actiontext/test/dummy/app/views/messages_mailer/notification.html.erb b/actiontext/test/dummy/app/views/messages_mailer/notification.html.erb
new file mode 100644
index 0000000000..e65604cecf
--- /dev/null
+++ b/actiontext/test/dummy/app/views/messages_mailer/notification.html.erb
@@ -0,0 +1 @@
+<div id="message-content"><%= @message.content %></div>
diff --git a/actiontext/test/dummy/config/routes.rb b/actiontext/test/dummy/config/routes.rb
index 1fc667e242..e7b45701e1 100644
--- a/actiontext/test/dummy/config/routes.rb
+++ b/actiontext/test/dummy/config/routes.rb
@@ -1,4 +1,7 @@
 Rails.application.routes.draw do
   resources :messages
-  # For details on the DSL available within this file, see https://guides.rubyonrails.org/routing.html
+
+  namespace :admin do
+    resources :messages, only: [:show]
+  end
 end
diff --git a/actiontext/test/integration/controller_render_test.rb b/actiontext/test/integration/controller_render_test.rb
new file mode 100644
index 0000000000..2e8812ed88
--- /dev/null
+++ b/actiontext/test/integration/controller_render_test.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class ActionText::ControllerRenderTest < ActionDispatch::IntegrationTest
+  test "uses current request environment" do
+    blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpg")
+    message = Message.create!(content: ActionText::Content.new.append_attachables(blob))
+
+    host! "loocalhoost"
+    get message_path(message)
+    assert_select "#content img" do |imgs|
+      imgs.each { |img| assert_match %r"//loocalhoost/", img["src"] }
+    end
+  end
+
+  test "resolves partials when controller is namespaced" do
+    blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpg")
+    message = Message.create!(content: ActionText::Content.new.append_attachables(blob))
+
+    get admin_message_path(message)
+    assert_select "#content-html .trix-content .attachment--jpg"
+  end
+end
diff --git a/actiontext/test/integration/mailer_render_test.rb b/actiontext/test/integration/mailer_render_test.rb
new file mode 100644
index 0000000000..e787379445
--- /dev/null
+++ b/actiontext/test/integration/mailer_render_test.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+class ActionText::MailerRenderTest < ActionMailer::TestCase
+  test "uses default_url_options" do
+    original_default_url_options = ActionMailer::Base.default_url_options
+    ActionMailer::Base.default_url_options = { host: "hoost" }
+
+    blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpg")
+    message = Message.new(content: ActionText::Content.new.append_attachables(blob))
+
+    MessagesMailer.with(recipient: "test", message: message).notification.deliver_now
+
+    assert_select_email do
+      assert_select "#message-content img" do |imgs|
+        imgs.each { |img| assert_match %r"//hoost/", img["src"] }
+      end
+    end
+  ensure
+    ActionMailer::Base.default_url_options = original_default_url_options
+  end
+end
diff --git a/actiontext/test/unit/content_test.rb b/actiontext/test/unit/content_test.rb
index f77f48df14..c00ad181a5 100644
--- a/actiontext/test/unit/content_test.rb
+++ b/actiontext/test/unit/content_test.rb
@@ -107,6 +107,15 @@ class ActionText::ContentTest < ActiveSupport::TestCase
     assert_equal "<blockquote><div>#{attachment_html}</div></blockquote>", content_from_html(html).to_html
   end
 
+  test "renders with layout when ApplicationController is not defined" do
+    html = "<h1>Hello world</h1>"
+    rendered = content_from_html(html).to_rendered_html_with_layout
+
+    assert_includes rendered, html
+    assert_match %r/\A#{Regexp.escape '<div class="trix-content">'}/, rendered
+    assert_not defined?(::ApplicationController)
+  end
+
   private
     def content_from_html(html)
       ActionText::Content.new(html).tap do |content|
diff --git a/actionview/lib/action_view/base.rb b/actionview/lib/action_view/base.rb
index 9ed1f5898c..d06c1287ef 100644
--- a/actionview/lib/action_view/base.rb
+++ b/actionview/lib/action_view/base.rb
@@ -151,7 +151,7 @@ module ActionView #:nodoc:
     # Specify whether rendering within namespaced controllers should prefix
     # the partial paths for ActiveModel objects with the namespace.
     # (e.g., an Admin::PostsController would render @post using /admin/posts/_post.erb)
-    cattr_accessor :prefix_partial_path_with_controller_namespace, default: true
+    class_attribute :prefix_partial_path_with_controller_namespace, default: true
 
     # Specify default_formats that can be rendered.
     cattr_accessor :default_formats