Mithral Armor is a uncommon armor in D&D 5e. It does not require attunement.
Armor (medium or heavy), uncommon
Mithral is a light, flexible metal. A mithral chain shirt or breastplate can be worn under normal clothes. If the armor normally imposes disadvantage on Dexterity (Stealth) checks or has a Strength requirement, the mithral version of the armor doesn't.