Catalyzes the NAD(P)H-dependent reduction of mycothione (the oxidized disulfide form of mycothiol) to mycothiol. 2 mycothiol + NADP(+) = H(+) + mycothione + NADPH 2 mycothiol + NAD(+) = H(+) + mycothione + NADH Binds 1 FAD per subunit. Homodimer. Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family.