Automatic Login in OS X Mountain Lion
Some users don’t care for the fact that OS X Mountain Lion is a multiuser operating system — and dislike having to log in when they start up their Mac. For those users, here’s a way to disable the login screen:
Open the Users & Groups System Preferences pane, select yourself in the list of users, and click the Login Options button below the list.
Choose the account you want to be logged in automatically from the Automatic Login pop-up menu.
To disable the logging-in requirement, you have to be an administrator, and you might need to unlock the Users & Groups System Preferences.
When you disable logging in, you also affect all the preferences set by anyone else who uses your Mac unless they log out of your account and log into theirs. (Yikes.) So if your Desktop pattern, keyboard settings, and so forth are different from those of someone else who uses your machine, those preferences won’t be properly reflected unless each of you has a separate, individual login account.
Even if you’re not worried about security, consider keeping logging in enabled if any other users have accounts on your machine, or if you don’t want just anyone to be able to turn on your Mac and see your personal stuff.
Note that only one account is allowed to use autologin. If another user wants to use this Mac, you need to choose Apple→Log Out, press Command+Shift+Q, or have Fast User Switching enabled. And if you’ve disabled automatic login in the Security System Preferences pane, you can’t enable it here.