During setup, peppermint does not allow the option to auto login.
You arrange auto login after installation like this:
Create a new user name from: Memu > preferences > Users and groups
(ie. user2) and a password,
then edit the file /etc/xdg/peppermint-ice/lxdm.conf (peppermint ice)
or file: /etc/xdg/peppermint/lxdm/lxdm.conf (peppermint)
You will have to do this as root, and alter the first line
#autologin=(whatever it is) to the new user name you created (user2)
by removing the # and deleting the default name (dgod) and adding your created user name.
Then uncomment (remove the #) from the second line, save, and reboot. That should bring it straight to the desktop with no login.
Add the new user to the sudoers file, or (s)he will not be able to sudo or su to root.