Carbon nanotubes (CNTs) are attractive candidates as nanofillers in reinforcing ordinary Portland cement (OPC) due to their superior mechanical properties. In this study, the engineering properties of CNT-OPC pastes were investigated with varied ultrasonication energy (UE) and CNT concentration. It was found that UE could effectively improve the aqueous dispersion of surface functionalized CNTs with the aid of a polycarboxylate-based cement admixture (PC). A PC to CNTs mass ratio of 8 is recommended for ensuring effective dispersion of CNTs and maintaining workability of CNT-OPC pastes under sufficient ultrasonication. Furthermore, the mechanical property results of the hardened pastes obtained from pre-notched beam tests revealed the existence of an optimal UE for achieving mechanically superior CNT-OPC pastes, which was found to be 50 J/mL per unit CNTs to suspensions weight ratio. The Young's modulus E, flexural strength σf, and fracture energy GF of CNT-OPC pastes were significantly improved compared to plain OPC pastes. These results clearly demonstrate the reinforcing effect of CNTs on cement pastes because they decrease the porosity and increase crack bridging capacity of cement pastes at nanoscale level, which will be complementary to conventional microfibers in reinforcing OPC pastes.