Does it stack with Mage Armor?
— Wackee (@WackeeSmite) October 4, 2017
The Defense fighting style requires you to wear armor. Mage armor requires you to not wear armor. They're mutually exclusive. #DnD https://t.co/znDXZWlHFx
— Jeremy Crawford (@JeremyECrawford) October 5, 2017
If one is wearing light armor and they have the defensive fighting style do still gain +1 to their AC if they then cast mage armor. Having mage armor over or under their armor the same way it mage armor is over or under one’s clothing.
“Mage Armor” isn’t an “armor”, Abraham — it can’t be optionally “over” or “under” what he caster is wearing; rather, it’s a mild protective field that provides modest protection for the caster. It literally says in the spell that it’s a magical field and that you can’t be wearing any armor at all to have it cast on you and it ends if you put armor on.
Mage Armor states that the target has to be unarmored. You can’t be wearing light armor and then cast Mage armor on yourself.