sudo vi /etc/lightdm/lightdm.conf.d/70-linuxmint.conf
Change this:
[SeatDefaults]
user-session=mate
allow-guest=false
To this:
[SeatDefaults]
user-session=mate
allow-guest=false
greeter-hide-users=true
greeter-show-manual-login=true
To see and apply your changes just restart light........