When you search for such question using google you get a lot of hits but all solutions assume you have at least one window.
But my question is just l
Two options:
Use System.Windows.Forms.Control.MousePosition, or p/invoke
[DllImport("user32.dll", CharSet=CharSet.Auto, ExactSpelling=true)]
public static extern bool GetCursorPos([In, Out] NativeMethods.POINT pt);
The first option already does the p/invoke for you. I'm not entirely sure it requires you have some UI splashed up, but I don't think so. Yes, its winforms and not wpf, but it really doesn't have anything to do with where its located at.
If you want to skip any dependencies on system.windows.forms.dll then check out more information about the second on pinvoke.net.