Nanostructured mixed phospho-olivine LiMn y Fe1−y PO4/C composites having different ratio of manganese and iron (0< y <1) are prepared by an easy, quick and cost effective hydrothermal synthesis in the presence of a cationic surfactant and their electrochemical behaviour as cathodes for Li-ion battery is demonstrated. A detailed study of the influence of Fe substitution with Mn on the structure, morphology and electrochemical performances of the resulting samples is performed by means of X-ray powder diffraction and Rietveld refinement of the diffraction patterns, inductively coupled plasma atomic emission spectroscopy, X-ray photoelectron spectroscopy, N2 physisorption at 77K, scanning and transmission electron microscopy, cyclic voltammetry and constant current charge–discharge cycling. The best performing LiMn y Fe1−y PO4/C composite sample is able to deliver a very stable cycling behaviour for several hundred cycles with good specific capacity values, high Coulombic efficiency and rate capability and is also demonstrated to work in a real Li-ion battery configuration versus a graphite anode. These improved electrochemical properties are ascribed to the kind of synthesis adopted that leads to nanostructured particles, homogeneously coated by a thin layer of carbon, which exerts its positive effect by improving electronic conductivity and suppressing Mn dissolution from the olivine structure during prolonged cycling.