This study evaluated the flexural properties of an experimental composite series functionalized with 5–40 wt% of a low-Na F-containing bioactive glass (F-series) and compared it to another experimental composite series containing the same amounts of the conventional bioactive glass 45S5 (C-series). Flexural strength and modulus were evaluated using a three-point bending test. Degree of conversion was measured using Fourier-transform infrared spectroscopy. Weibull analysis was performed to evaluate material reliability. The control material with 0 wt% of bioactive glass demonstrated flexural strength values of 105.1–126.8 MPa). In the C-series, flexural strength ranged between 17.1 and 121.5 MPa and was considerably more diminished by the increasing amounts of bioactive glass than flexural strength in the F-series (83.8–130.2 MPa). Analogously, flexural modulus in the C-series (0.56–6.66 GPa) was more reduced by the increase in bioactive glass amount than in the F-series (5.24–7.56 GPa). The ISO-recommended “minimum acceptable” flexural strength for restorative resin composites of 80 MPa was achieved for all materials in the F-series, while in the C-series, the materials with higher bioactive glass amounts (20 and 40 wt%) failed to meet the requirement of 80 MPa. The degree of conversion in the F-series was statistically similar or higher compared to that of the control composite with no bioactive glass, while the C-series showed a declining degree of conversion with increasing bioactive glass amounts. In summary, the negative effect of the addition of bioactive glass on mechanical properties was notably less pronounced for the customized bioactive glass than for the bioactive glass 45S5; additionally, mechanical properties of the composites functionalized with the customized bioactive glass were significantly less diminished by artificial aging. Hence, the customized bioactive glass investigated in the present study represents a promising candidate for functionalizing ion-releasing resin composites.