Upgrade Install of macOS Mountain Lion

Move up to Mountain Lion without losing your personal data

Mountain lion

Geoffrey Kuchera / Shutterstock

There are several ways to install macOS Mountain Lion (10.8). This guide will show you how to perform an upgrade install, which is the default installation method. You can also perform a clean install, or install the OS from other media, such as a USB flash drive, a DVD, or an external hard drive. Those options are covered in other guides.

OS X Mountain Lion Installer app
Coyote Moon, Inc.

Mountain Lion is the second version of macOS that can only be purchased through the Mac App Store. The upgrade install process lets you install Mountain Lion over your existing version of macOS and still retain all of your user data, most of your system preferences, and most of your applications. You may lose some apps that cannot run on Mountain Lion. The installer may also change some of your preference files because certain settings are no longer supported or are incompatible with some features of the new OS.

We recommend backing up your current system before starting the upgrade process. You can create a Time Machine backup, a clone of your startup drive, or just back up your most precious files.

What You Need to Perform an Upgrade Install of OS X Mountain Lion

  • A copy of the Mountain Lion installer, which is available from the Mac App Store. You must be running Snow Leopard or later to access the Mac App Store, but you don't have to install Lion before you install Mountain Lion. Mountain Lion will install correctly as long as you're running OS X Snow Leopard or later.
  • A destination volume for the installation. The Mountain Lion installer can work with internal drives, SSDs (Solid State Drives), or external drives with USB, FireWire, or Thunderbolt interfaces. Basically, any bootable device will work, but because this is a guide to an upgrade install, the target volume must be running OS X Lion or earlier. If your Mac doesn't meet this requirement, then the Clean Install guide is a better choice for you.
  • A minimum of 8 GB of free space, but more space is better, of course.
  • A minimum of 650 MB of free space for the Recovery HD volume. This is a hidden volume that is created during the installation. The Recovery HD volume contains utilities to repair drives and to re-install the OS if you have problems with a drive.

If you have everything lined up with current backups in place, you can proceed to the upgrade process.

How to Upgrade Install macOS Mountain Lion

OS X Mountain Lion Installer with destination disk selection.
The Mountain Lion installer selects your current startup drive as the target for the installation (the Show All Disks button is only visible if there are multiple drives connected to your Mac.). Coyote Moon, Inc.

This guide will take you through an upgrade install of macOS Mountain Lion. The upgrade will replace the version of macOS your Mac is currently running, but it will leave your user data and most of your preferences and apps in place. Before you begin the upgrade, make sure you have a current backup of all your data. While the upgrade process shouldn't cause any problems, it's always best to be prepared for the worst.

  1. Launch the Mountain Lion installer. When you purchase Mountain Lion from the Mac App Store, it will be downloaded and stored in the Applications folder; the file is called Install OS X Mountain Lion. The download process may also create a Mountain Lion installer icon in the Dock for easy access.

  2. Close any applications that are currently running on your Mac, including your browser and this guide. If you need to consult these instructions, you can print the guide or use a mobile device to read them.

  3. With the Mountain Lion installer window will open, select Continue.

  4. The license agreement will appear. You can read the terms of use or select Agree to continue.

  5. A new dialog box will ask if you've read the terms of the agreement. Select Agree.

  6. By default, the Mountain Lion installer selects your current startup drive as the target for the installation. If you want to install Mountain Lion on a different drive, select Show All Disks, select the target drive, then select Install.

  7. Enter your administrator password, then select OK.

  8. The Mountain Lion installer will start the installation process by copying the necessary files to the selected destination drive. The amount of time this will take depends on how fast your Mac and the drives are. When the process is complete, your Mac will restart automatically.

  9. After your Mac restarts, the installation process will continue. A progress bar will display to give you an idea of how much time is needed to complete the installation. When the installation is complete, your Mac will once again restart.

If you use multiple monitors, be sure to have all of the monitors turned on. During the installation, the progress window may display on the secondary monitor instead.

How to Set up macOS Mountain Lion

Upgrade Install OS X Mountain Lion
Screenshot 

When Mountain Lion is installed, either the log-in screen or the Desktop will appear, depending on whether you previously had your Mac configured to require log-in. If you did not have an Apple ID set up for your current OS, the first time your Mac starts up with Mountain Lion it will ask you to supply an Apple ID and password.

  1. Enter your Apple ID and password and select Continue, or skip this step by selecting Skip.

  2. The Mountain Lion license agreement will appear. This includes the macOS license, the iCloud license, and the Game Center license. Read the information if you'd like, then select Agree. Apple will ask you to confirm the agreement. Select Agree once again.

  3. If you don't already have iCloud set up on your Mac, you'll be given the option to use the service. If you wish to use iCloud, place a checkmark in the Set Up iCloud on This Mac checkbox, then select Continue. If you don't want to use iCloud, or you'd rather set it up later, leave the checkbox empty and select Continue.

  4. If you choose to set up iCloud now, you'll be asked if you want to use Find My Mac, a service that can locate your Mac on a map if you ever lose or misplace it. Make your selection by placing or removing a checkmark, then select Continue.

  5. The installer will finish and present a Thank You display. Select Start Using Your Mac.

Update the Mountain Lion Software

Before you get busy with macOS Mountain Lion, you should run the Software Update service. This will check for updates of the OS and many supported products, such as printers, that are connected to your Mac and may need updated software to work correctly with Mountain Lion.

You can find Software Update under the Apple menu.