This present work introduces a strategically engineered P2- type NaMg0.25Mn0.75O2 (NNMO) cathode material by sol-gel auto-combustion techniques with optimized cation substitution (Mg2+ for Mn4+), which enhances structural stability and high-temperature electrical performance. The X-ray diffraction (XRD) pattern and Raman spectra reveal the P2- type hexagonal crystal structure (space group P63/mmc) and three sharp Raman active phonon modes A1g, E2g, and E1g at 595, 480, and 390 cm−1, respectively. The ac electrical transport properties i.e., impedance, conductivity, dielectric, and modulus spectra of NaMg0.25Mn0.75O2 investigated at different temperatures from 298 to 523 K (ΔT = 50 K) and frequency range between 100 Hz and 1 MHz. It has been discovered from impedance and conductivity that the NMMO pellet sample contains short-range (AC) and long-range (DC) charge carrier movements, with the former predominating at low temperatures and the latter at high temperatures. The variation of frequency exponent (n) decreases with temperatures which reveals the correlated barrier hopping (CBH) conduction process where the charge carriers (electrons or holes) hop between localized states over a potential energy barrier. The dc activation energy (Ea) was calculated at ∼0.48 eV using Arrhenius plots. High dielectric constant (εr)∼5.4 × 106 and low loss (δ)∼23 were obtained at high temperature (523 K) and low frequency (50 Hz) region which recommends its high energy storage capacity at room and high temperatures. The thermal robustness, stable dielectric response, and favorable ac electrical conduction of NMMO demonstrated its possible technological application as a capacitor in high-temperature power electronics and next generation energy storages applications.