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Utilizing metal–organic frameworks (MOFs) as reinforcing fillers for polymer composites is a promising strategy because of the low density, high specific modulus, and tunable aspect ratio (AR). However, it has not been demonstrated for the MOF-reinforced polymer composite using MOFs with high AR and polymer-grafted surface, both of which are extremely important factors for efficient load transfer and favorable particle–matrix interaction. To this end, we designed an MOF–polymer composite system using high AR MOF PCN-222 as the mechanical reinforcer. Moreover, we developed a synthetic route to graft poly(methyl methacrylate) (PMMA) from the surface of PCN-222 through surface-initiated atomic transfer radical polymerization (SI-ATRP). The successful growth of PMMA on the surface of PCN-222 was confirmed via proton nuclear magnetic resonance and infrared spectroscopy. Through thermogravimetric analysis, the grafting density was found to be 0.18 chains/nm2. The grafted polymer molecular weight was controlled ranging from 50.3 to 158 kDa as suggested by size exclusion chromatography. Finally, we fabricated MOF–polymer composite films by the doctor-blading technique and measured the mechanical properties through the tension mode of dynamic mechanical analysis. We found that the mechanical properties of the composites were improved with increasing grafted PMMA molecular weight. The maximum reinforcement, a 114% increase in Young’s modulus at 0.5 wt % MOF loading in comparison to pristine PMMA films, was achieved when the grafted molecular weight was higher than the matrix molecular weight, which was in good agreement with previous literature. Moreover, our composite presents the highest reinforcement measured via Young’s modulus at low weight loading among MOF-reinforced polymer composites due to the high MOF AR and enhanced interface. Our approach offers great potential for lightweight mechanical reinforcement with high AR MOFs and a generalizable grafting-from strategy for porphyrin-based MOFs.