Polypropylene/montmorillonite (iPP/MMT) has been investigated as an alternative to isotactic polypropylene (iPP) as a matrix for glass mat thermoplastic (GMT) composites. The melt viscosity increased significantly on MMT addition, but iPP/5.9wt% MMT-based hybrid GMT preforms were fully impregnated after 30 s of compression molding, which is consistent with GMT industrial cycle times. The flexural modulus and strength increased monotonically with MMT content in consolidated hybrid GMT specimens prepared by compression molding, although the impact strength decreased. Moreover, the increases in the flexural modulus were greater than predicted from a conventional rule of mixtures taking into the measured Young’s moduli of the iPP/MMT matrices. Possible reasons for this are discussed.