Ubuntu since 14.04 switches to systemd to manage services. Time to learn something new.

Key to systemd is the command systemctl

Disable a service (so that it does not start when system reboots):
sudo systemctl disable mongodb

Enable a service (for automatic startup when system reboots):
sudo systemctl enable mongodb

Start / Stop a service:
sudo systemctl start mongodb
sudo systemctl stop mongodb

Further helpful tweaks for systemd on this page. See also this former post about upstart and systemv runlevels.