Why do I have mild pain under my eyebrows?

Eye Strain. One possible cause for mild pain below the eyebrows is eyestrain. You should see your eye md to determine possible need for glasses. Another possible source for eyebrow pain is inflammation in the sinuses, so if your eye md can't find an explanation you should see and ENT md.
Sinus. Sounds like sinus disease. Try otc sinus medication and go to ENT doctor of no relief.