Sometimes it's easier to remember one password, and in this case Apple enable you to use your iCloud password to unlock your Mac. Naturally, you'll need to already have access to your Mac's settings for security purposes.
By default if you've setup your Mac using iCloud, you'll already be using your iCloud password to login to your Mac. If for any reason that has changed, or you wish to re-enable the option, read on.
How to unlock Mac with iCloud: System preferences
To change your login options, you'll need to get into system preferences, which can be done by clicking the Apple logo at the top left-hand side of your Mac and selecting system preferences.
In the new pop-up window click on User & Groups, which will present you with the users who are currently able to login on your Mac. Read next: How to use iCloud Drive for Mac, iPhone & iPad.
Select the account you're currently logged in as, and press on Change Password. You'll now be presented with the option to either use your iCloud password or a separate password.
Click on the iCloud password option and type in your iCloud password to confirm the selection. You're now using your iCloud (again) to login to your Mac. This is of course fully reversible by doing the exact same steps so that you use a separate password from your iCloud one to unlock the Mac.
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