Can a mundane shield be used with Mage Armor?

Question

Mage armor:

You touch a willing creature who isn't wearing armor, and a protective magical force surrounds it until the spell ends. The target's base AC becomes 13 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action.

A shield grants a +2 bonus to AC, and is in the list of equipment that can be donned and doffed. Can the spell be cast on someone wielding a shield, and would it end if cast on a person who then dons a shield?

As a sidenote, while this question shares similarities with a previously asked question (Does Mage Armor stack with Shield?), that question deals primarily with whether untyped bonuses can add onto mage armor, and whether the two spell effects can combine, whereas this question queries a specific limitation of the mage armor spell (cannot work with armour) and whether a shield counts as armour in this regard.

Answer 1

Yes, as per a Sage Advice column covering this specific question.

Can you use a shield with mage armor? Mage armor works with a shield. Shields are grouped with armor in the equipment rules in the Player’s Handbook, but various game features distinguish between the armor you wear and a shield you wield. Take a look at the monk’s Unarmored Defense feature and compare it to the barbarian’s version to see what I mean. In the monk’s version, you must both forgo wearing armor and forgo wielding a shield if you want to benefit from the feature, whereas a barbarian must only forgo wearing armor.

Answer 2

Yes, because a shield is not armour

The shield in the equipment chapter is in a different category than light, medium, and heavy armour, and in the classes section shields are listed as a separate proficiency than each of the light, medium, and heavy armour.

Also PH page 14, Armor Class, mention wearing armour and carrying a shield several times. Note that you wear armour and you carry a shield.

In addition, the Sage Advice column has a collection of frequently asked questions (current as of 9.Apr.2016), and in it there is a section on determining AC.

These methods—along with any others that give you a formula for calculating your AC—are mutually exclusive; you can benefit from only one at a time.

…

What about a shield? A shield increases your AC by 2 while you use it…[k]eep in mind that some AC calculations, such as a monk’s Unarmored Defense, prohibit the use of a shield.

Seeing as how mage armor gives you another way to determine AC (13 + Dex mod), and doesn't prohibit the use of a shield, it seems safe to allow them to be used in conjunction.

Answer 3

Yes, Mage Armour and a shield stack.

The armour that the Mage Armour spell mentions is body armour, like plate or a breastplate. Mage Armour acts as a suit of armour, so it doesn't work with body armour. A shield is not body armour, and so it can be used at the same time as Mage Armour, and it doesn't cause Mage Armour to go away.

Answer 4

No; a shield is armour and/or sets base AC.

In PHB (p. 145), the shield is in a table titled "Armour". The groups in the box are - light armour, medium armour, heavy armour, and shields. This suggests that if light armour is a kind of armour then so is a shield.

PHB p. 144 shows that the divide between shields and armour is at the very least blurred. According to the section on armour proficiency, using shields without proficiency is possible - "Anyone can put on a suit of armour or strap a shield to an arm", but carries no disadvantage.

The next section states that the armour and shields you wear set your base AC. Here the separation seems to undermine my previous point, but even if shields are not armour, they set base AC rather than being a bonus over it, so since Mage Armour sets the base AC using a new calculation, it doesn't take shields into account.