<p>This study analyzes the stability of mixed convective Jeffrey fluid flow in an anisotropic porous medium under local thermal non-equilibrium (LTNE), incorporating dissimilar fluid and solid phase thermal transport. The governing equations are derived within the Jeffrey–Darcy model, accounting for momentum and energy conservation under the Oberbeck–Boussinesq approximation. Directional variations in permeability and thermal conductivity are incorporated to reflect anisotropic porous media behavior. A linear stability analysis is conducted using normal mode decomposition, transforming the system into a Generalized Eigenvalue Problem (GEP). This GEP is numerically solved using the Chebyshev collocation method, ensuring spectral accuracy. The critical thresholds in terms of the wave number, frequency of oscillations, and Darcy–Rayleigh number are evaluated with respect to key parameters: thermal diffusivity ratio, porosity modified conductivity ratio, horizontal pressure gradient, Jeffrey parameter, inter-phase heat transfer coefficient, and both mechanical and thermal anisotropies. The analysis reveals that the Jeffrey parameter destabilizes the system by narrowing the stable regime and promoting oscillatory convection, while mechanical anisotropy enhances instability region whereas both fluid and solid thermal anisotropies delays the onset of convection. The interphase heat transfer coefficient and applied horizontal pressure gradient are found to delay instability, requiring stronger thermal driving for convection onset. Non-linear stability analysis using the method of lines confirms the linear predictions, showing the emergence of transient convection cells and isotherm structures. The horizontal pressure gradient introduces subcritical instabilities, indicating metastable regimes due to delayed fluid–solid thermal coupling. These findings advance the understanding of convective stability in anisotropic porous media with Jeffrey fluids for many engineering applications.</p>