This research introduces a compact, self-isolated, 12 × 12 multiple-input multiple-output (MIMO) antenna array designed for 5G mobile applications, operating within the 3.5 GHz band (3.42–3.62 GHz). The array consists of two distinct sets of six antenna elements—inverted U-shaped and T-shaped structures, each incorporating two circular and I-shaped strips—arranged symmetrically within the smartphone chassis. Each antenna element measures 15 × 5 mm² (0.17λ_o × 0.06λ_o), where λ_o represents the free-space wavelength at 3.5 GHz. These elements function simultaneously as radiators and isolators, achieving high isolation levels. Additionally, the 12-MIMO antenna elements, designed to be self-isolated, are fabricated on both sides of two compact FR-4 substrates, positioned orthogonally to the ground substrate. This perpendicular configuration augments the self-isolating mechanism. The scattering parameters (s-parameter) findings demonstrate significant decoupling measuring under −19 dB across neighboring 12-MIMO elements. Moreover, the MIMO performance metrics, including channel capacity loss (CCL), total active reflection coefficient (TARC), diversity gain (DG), and the envelope correlation coefficient (ECC), are presented to be below 0.08 bits/s/Hz, −10 dB, 9.97 dB, and 0.006 consecutively. The significant isolation and performance metrics results notably indicate that the presented 12-MIMO antenna system is well-suited for 5G communication systems.