How to release mouse from VirtualBox?

to return ownership of keyboard and mouse to your host operating system, VirtualBox reserves a special key on your keyboard for itself: the "host key". By default, this is the right Control key on your keyboard; on a Mac host, the default host key is the left Command key.

Have you set your release key to something else and forgotten about it? I believe the default key is the right ctrl key, but you can also set your own.

For a troublesome Vagrant-generated guest machine running a Linux server distro without graphics which doesn't respond to the other solutions here: With the [text] console of the guest machine (where it is still possible to type), shutdown the guest with sudo shutdown -h now ; the guest machine halts and removes itself from the screen, which returns mouse and keyboard control to the the host PC (in my case I was having to login before I could issue the shutdown command; reminder: default vagrant credentials are vagrant/vagrant)