Incorporating graphene-based nanomaterials into thermosetting resins is challenging at an industrial-scale. To address this issue, we prepared a styrene masterbatch containing chemically modified graphene oxide (mGO) and added it to unsaturated polyester and vinyl ester resins via simple mechanical mixing to generate homogeneous mGO/resin dispersions. For comparison, oven-dried or freeze-dried mGO was also blended into resin using the same mechanical mixing conditions. At low mGO loading levels of 0.02-0.08 wt %, composites made with oven-dried or freeze-dried mGO show the highest increase in fracture toughness but also the most severe decrease in flexural strength, whereas composites prepared from the mGO masterbatch show the best dispersion homogeneity and better retention of flexural strength while exhibiting only slightly less increase in toughness. The masterbatch process offers an economical way of producing high-quality mGO/resin dispersions.