A modular vault assembly including a plurality of vault modules variable in number and including a primary vault and one or more secondary vaults. The secondary vaults are interconnected in successively adjacent relation and a leading secondary vault is connected adjacent to the primary vault. The primary vault and each of the secondary vaults respectively include a master door and a slave door having a locking mechanism disposable in a locked and unlocked orientation. The locking mechanism of the leading secondary vault is manually positioned in the unlocked orientation from an interior of said primary vault and each of said locking mechanisms of a remainder of the secondary vaults is manually positioned, successively, in the unlocked orientation from an interior of a preceding, next adjacent one of the plurality of secondary vaults. Each master and slave door includes a sensor assembly indicating the locked orientation of a corresponding locking mechanism.