How to lock screen with keyboard on Linux Mint (LXDE)
I use mint10 where you go to menu-> preferences -> keyboard shortcuts.
I have not been able to find any gui lxde and do not have away of testing these methods.
LXDE:Questions,
Search to Index 3. mouse/keyboard,for mint 11 the file is mintlxde-rc.xml.
From Stray notes,
From Sourceforge,
From openbox/org,
I hope something helps.
In Linux Mint 18.3 (Sylvia) GUI, go to Menu -> Preferences -> Keyboard -> Shortcuts.
Expand the System tab and you should see a Lock Screen shortcut setting.
The default shortcut to lock the screen is Ctrl+Alt+L.