A single phase of LiMn1−x Fe x PO4(OH) with the tavorite structure is synthesized by a H+/Li+ exchange reaction of Mn1−x Fe x PO4·H2O precursors and LiNO3 in the composition range 0≤ x ≤0.3. The materials are characterized by X-ray diffraction, thermogravimetry–differential thermal analysis, scanning electron microscopy, and charge–discharge measurements. Rietveld refinement results of synchrotron X-ray powder diffraction data reveal that LiMn1−x Fe x PO4(OH) forms a continuous solid solution over the entire composition range with a triclinic structure (space group: P–1). A linear reduction in the unit cell dimensions a and b and a linear increase in the cell parameter c are accompanied by a local change in the bonding geometry of M 3+O6 and LiO6 octahedra with increasing iron content. Iron-substituted phases exhibit an improved charge–discharge performance with a 30% increase in capacity for voltages in the range of 2.0–4.6V as the iron content increases from x =0.0 to 0.3. The improved electrochemical properties and thermal stability reveal a correlation with distortion relaxation in the local geometry caused by partial substitution of Mn3+ by Fe3+ in LiMnPO4(OH).