In this study, a high-performance fiber-reinforced cementitious composite containing wollastonite powder was developed for the flexural strengthening of reinforced concrete beams. Despite recent advances in incorporating mineral admixtures into cementitious industries, the exact physical-chemical synergistic mechanism of wollastonite and its role in improving the interfacial transition zone between fibers and the matrix in strengthening sheets remain a research gap. To this end, the effect of 0% to 20% wollastonite substitution on the mechanical behavior of the composite was evaluated. Experimental results indicated that the optimum substitution ratio of 20%, owing to the acicular structure of wollastonite and its microstructural role in enhancing the matrix, provides a desirable fiber-like performance and pseudo-strain-hardening behavior. The main innovation of this study lies in clarifying the microstructural mechanism of wollastonite on the integrity of the cementitious matrix, as well as developing a validated numerical model to predict the flexural behavior of beams strengthened with these sheets and glass fiber-reinforced polymer bars. Parametric studies demonstrated that increasing the thickness of the strengthening sheet, as well as the diameter and number of polymer bars, plays a decisive role in improving the flexural performance of the strengthened system. Ultimately, the application of this strengthening system led to a significant increase in the load-carrying capacity of the beams from 120.05 to 267.41 kN and the energy absorption capacity from 1479.35 to 2921.84 J, demonstrating the high potential of this composite as an efficient, sustainable, and innovative solution for the rehabilitation and strengthening of concrete structures.