Composites with general stoichiometry Na3V2−xMnx(PO4)3/C (0 ≤ x ≤ 0.7) have been evaluated as cathode materials in sodium-ion batteries. The sol-gel method here employed favors the formation of a homogeneously dispersed carbon conductive phase. XRD patterns show a limited solubility of Mn in the NASICON structure. The substitution of 0.3–0.5 manganese per formula unit promotes the formation of a homogeneous composite. XPS spectra evidence an effective substitution of V3+ by Mn3+. Galvanostatic cycling of sodium half-cell reveal a plateau at 3.4 V ascribable to the V4+/V3+ redox couple. A small and reversible plateau at ca. 3.85 V is also observed for x ≥ 0.3, and ascribed to the activation of the V5+/V4+ redox couple, according to XPS spectroscopy. Ex-situ XRD patterns NayV1.7Mn0.3(PO4)3 evidence the occurrence of a reversible two phase mechanism of sodium insertion. An optimized performance is achieved for Na3V1.7Mn0.3(PO4)3/C, reaching a capacity value of 104 mA h g−1 at C/2 and 92 mA h g−1 at 2C. It is ascribed to the optimal morphology, leading to low internal resistance and favorable electrode-electrolyte interphase.