We investigate the mechanical properties of pi-conjugated polymeric materials composed of regioregular poly(3-hexylthiophene) (P3HT) and fullerene C-60 using coarse-grained molecular dynamics simulations. Specifically, we perform tensile simulations of P3HT:C-60 composites with varied degrees of polymerization and C-60 mass fractions to obtain their stress-strain responses. Decomposition of a stress tensor into kinetic energy and virial contributions indicates that the tensile moduli of the pure P3HT samples are greatly dependent on nonbonded interactions and on bonded interactions associated with bond stretching, while the addition of C-60 leads to an increase in the tensile modulus originating from enhanced nonbonded interactions associated with C-60. Additionally, the tensile strength of the P3HT:C-60 samples correlates well with molecular chain entanglements, which are characterized by the average number of kinks per chain obtained from primitive path analysis. We also find that the upper and lower yield points characterizing strain softening become more pronounced with an increasing C-60 mass fraction. Persistent homology analysis indicates that the emergence of the yield points correlates well with the coalescence of microvoids in the course of tensile deformation, resulting in the generation of larger voids. These results provide a fundamental understanding of the molecular determinants of the mechanical properties of pi-conjugated polymer-fullerene composites, which can also help to interpret and predict the mechanical properties of other polymer composites containing fullerene.