Hydrothermal microwave-assisted synthesis of Mn-substituted Na_3+xV_2–yMn_y(PO₄)₂F₃ positive electrode materials for Na-ion batteries resulted in phase-pure solid solutions with y ≤ 0.6. According to joint Rietveld refinement from the X-ray and neutron powder diffraction data, the solid solution adopts a highly disordered tetragonal I4/mmm structure [a = 6.4695(1) Å, c = 10.6402(3) Å for Na_3.6V_1.4Mn_0.6(PO₄)₂F₃]. Electron energy loss spectroscopy (EELS) revealed the Mn²⁺ and V³⁺ oxidation states, whereas the charge compensation for the heterovalent V³⁺ → Mn²⁺ substitution occurs by a corresponding increase in the Na content. Electrochemical tests on carbon-coated Na_3.6V_1.4Mn_0.6(PO₄)₂F₃ revealed the ability to reversibly (de)intercalate Na⁺ in the potential range of 2.5-4.6 V versus Na/Na⁺ with ∼108 mA h/g discharge capacity, ∼3.7% volume change, and >50% capacity retention at 10 C rate. Post-mortem EELS analysis revealed that the redox process up to 3.8 V is dominated by the Mn²⁺/Mn³⁺ pair, whereas in the 3.8-4.6 V region, the V³⁺/"V⁴⁺" pair is active where "V⁴⁺" is a result of disproportionation into V³⁺ and V⁵⁺. A promising application of the Mn-doped materials as positive electrodes in sodium-ion batteries is demonstrated by a steady-functioning Na_3.6V_1.4Mn_0.6(PO₄)₂F₃/hard carbon full cell with the specific energy of 312 Wh per kg of cathode.