This setting is needed for nemo to accept tilix as default terminal:

gsettings set org.cinnamon.desktop.default-applications.terminal exec tilix

It’s cinnamon because nemo is the default filemanager of Linux Mint and not the standard fm for Ubuntu which is nautilus.

This command only sets tilix as Ubuntu’s default terminal, not for nemo (if used):

gsettings set org.gnome.desktop.default-applications.terminal exec 'tilix'

If the following error message appears:

GLib-GIO-Message: Using the 'memory' GSettings backend.  Your settings will not be saved or shared with other applications.

This is often caused when anaconda is the first in the path. The system python (or probably a python 2.7 version) needs to be the first in the path. Be careful when doing something like export PATH=/usr/lib/bin. This deletes most commands from the path and a simple source ~/.bashrc or source ~/.zshrc won’t restore the complete path.