Cation-ordered Li[Ni_1/2Mn_3/2]O₄ with a P4₃32 space group (CO-LNMO) and “cation-disordered” (CDO) LNMO are thought to be the state-of-the-art materials for lithium-ion batteries. However, in contrast to CO-LNMO, the crystal structure and electrochemical reaction scheme of CDO-LNMO are not fully understood. We have measured the muon-spin rotation and relaxation (μSR) spectra for samples of both CO-LNMO and CDO-LNMO, in particular at their magnetic transition temperatures (T_C) below 130 K. The weak transverse field (wTF) μSR measurements reveal that the range of T_C for the CDO-LNMO sample is very large (ΔT_C ∼ 55 K) compared with that for the CO-LNMO sample (ΔT_C < 5 K). This suggests an inhomogeneous cation distribution in the CDO-LNMO sample, because the sample consists of multiple phases with different T_C. Based on the wTF-μSR result for stoichiometric LiMn₂O₄, we have proposed that CDO-LNMO is a mixture of Li[Ni_1/2−ωMn_3/2+ω]O₄ and LiMn₂O₄.