Keychain Access, a built-in application for Mac, can also be used to lock our screen besides its main functionality to keep our password protected. By following these steps, you will be able to see a lock icon stuck on your menu bar that can assist you in locking up your screen.
1 – Open Keychain Access (if you don’t know where it is, use Spotlight)
2 – Keychain Access → Preferences…
3 – Under General section, tick checkbox ‘Show Status in Menu Bar’
4 – A Lock icon should now appear on your menu bar. Click on it to see what it can do (obviously, locking screen is on of its features)