LiMn2–xNixO4 spinel phases, with their almost flat electrochemical curves composed of two plateaus around 4.7 V vs Li+/Li separated by a voltage difference ΔV of 20–60 mV, are good candidates for high power applications. The Ni/Mn order is one of the key parameters in understanding the electrochemical curve shape. In this work, the Ni/Mn order in the nickel-rich region of the spinel LiMn2–xNixO4 solid solution (0.38 ≤ x ≤ 0.50) has been investigated using time-of-flight powder neutron diffraction (TOF-PND) and density functional theory (DFT) calculations. For LiMn2–xNixO4 solid-solution samples prepared between 700 and 900 °C, Ni/Mn ordering was found to be retained to room temperature by systematic broadening of diffraction peaks with hkl indexes of mixed even/odd parity. This broadening is due to the increasing density of a planar defect called antiphase domain boundaries (APBs). DFT calculations performed on several Ni/Mn defective configurations and TOF-PND Rietveld refinement indicate that the {100} orientation of the APB boundary is the most probable. Hence, in the whole composition range, a unique ordered spinel phase within the space group P4332, with a single hkl-dependent parameter to represent the APB crossing probability, gives a measure of the Ni/Mn order coherence length. We show that this defect density is driven by the synthesis temperature and the nickel content of the spinel phase. A correlation between the synthesis condition effect on the local ordering and the voltage profile is given for two Ni/Mn initial ratios (0.4/1.6 and 0.5/1.5). The influence of the synthesis temperature on these two compositions is drastically different: for LiMn1.6Ni0.4O4, with a similar APB domain size whatever the temperature, only a slight variation of ΔV is observed. Reversely, for LiMn1.5Ni0.5O4, a strong increase of the ΔV with the synthesis temperature is evidenced, concomitant with a decrease in the APB domain size and the Ni content.