I can’t say there is a global setting but when I start a new image my Inkscape does start with all the units set to mm. I am using Inkscape 0.92 so these settings may be different than yours.
These are the settings I have set. I also change the document properties whenever I open an SVG that has been saved as units other than mm.
In the file menu file/document properties you can set the “Display units” and you can set the page sizing with “Custom Size” to use your units of choice.
Also in the Preferences you can make more changes. edit/preferences/interface/grids has two places to set units. Also in edit/preferences/behaviour/steps you could change more units although I still have these set to px but use mm for all my other settings and when I look in the XML editor I do not see px mentioned and all units appear to be mm.