Durability and service life of cementitious materials are seriously affected by their low flexural strength and high brittleness. Herein, we developed a novel polymer-modified cementitious composite (PMCC) with enhanced mechanical performances by introducing acrylic acid (AA) and 2-acrylanmido-2-methylpropanesulfonic acid (AMPS) monomers into the cement matrix to form a strong polymer-cement network via in situ polymerization. By optimizing the mass ratio of AA-AMPS copolymer to cement, PMCC with a tunable and enhanced mechanical performance can be achieved. Samples with 3.0 wt% AA-AMPS copolymer exhibited the highest increase of 81.1% in flexural strength while retained adequate compressive strength under the standard curing. The strong chemical interactions between Ca and AA-AMPS copolymer chains that can help form an interwoven polymer-cement network in the cement matrix contributed to this enhanced flexural strength. In fact, drawbacks including large pore size, easy crack, and brittleness can be surmounted effectively via the construction of a polymer-cement network in the cement matrix, then mechanical enhancement can be obtained. This work paves a new way to develop a unique PMCC with high flexural strength via the in situ polymerization of monomers within the cement matrix.