Magnetic nanoparticles are widely explored in biomedical applications, particularly as MRI contrast agents and for magnetic hyperthermia. However, their photothermal capabilities under near-infrared (NIR) irradiation remain underexplored in realistic, tissue-like environments. This study provides a comprehensive assessment of ultrasmall Fe₃O₄ nanoparticles (9.23 ± 2.97 nm) in 3D agarose-based tissue-mimicking phantoms, integrating their imaging and photothermal properties under clinically relevant conditions. Photothermal performance was tested under 850, 970, and 1100 nm NIR light, with 970 nm showing optimal efficiency (71.59%) and a penetration depth of 2.1 cm. With a high saturation magnetization of approximately 52.4 emu/g, the nanoparticles were evaluated as MRI contrast nanoagents, showing notable T1–T2 contrast enhancement across various concentrations. Their performance was systematically compared with the commercial agent Gadovist through magnetic resonance relaxometry, high-field preclinical MRI at 11.7 T, and clinical MRI at 1.5 T, providing a comprehensive assessment across multiple imaging platforms and concentration ranges. While this study does not include biological in vitro or in vivo models, the use of phantoms replicating tissue optical and thermal properties, combined with clinical imaging systems and safety-compliant irradiation, creates a high-fidelity platform for translational evaluation. These results support the development of dual-mode theranostic platforms and lay the groundwork for future in vivo studies of MRI-guided photothermal cancer therapy.