Plasmonic nanostructures have demonstrated great promise for their potential in treating solid tumors; however, the crucial ability to efficiently track the uptake of these particles within tumors before, during, and after treatment is still lacking. Engineering a compact, near-infrared plasmonic nanostructure with integrated image-enhancing agents for combined imaging and therapy is an essential nanomedical challenge. Core-shell Au/SiO2/Au nanomatryoshkas (NM) are a highly promising nanostructure for hosting either T1 MRI or fluorescent contrast agents with a photothermal therapeutic response in a compact geometry. Here, near-infrared-resonant NM are investigated, and simultaneous contrast enhancement for both T1 magnetic resonance imaging (MRI) and fluorescence optical imaging (FOI) is demonstrated by encapsulating both types of contrast agents in the internal silica layer between the Au core and shell. DOTA chelates are used to entrap Gd(III) and Fe(III) ions to form Gd-NM and Fe-NM, respectively. This method of T1 enhancement is even more effective for Fe(III), a potentially safer contrast agent compared to Gd(III). Fe−NM-based contrast agents are found to have relaxivities 2× greater than those found in the widely used gadolinium chelate, Gd(III) DOTA, providing a practical alternative that would eliminate Gd(III) patient exposure. This dual-modality nanostructure can enable not only tissue visualization with MRI but also fluorescence-based nanoparticle tracking for quantifying nanoparticle distributions in vivo, in addition to a near-infrared photothermal therapeutic response. Lastly, Gd2O3-NM, an alternative core-shell NM comprising a Au core, Gd2O3 spacing layer, and outer Au shell is investigated. This nanoparticle maintains the high T1 relaxivity in terms of Gd(III) concentration compared to Gd-NM, but can load a significantly greater amount of Gd(III). This combination of high relaxivity and increased Gd(III) concentration per NP in Gd2O3-NM resulted in a 21 x enhancement of the T1 relaxivity in terms of NP concentration compared to Gd-NM. The stability of Gd(III) within Gd2O3-NM and Gd-NM in acidic environments was also investigated. Minimal leaching of Gd(III) from Gd2O3-NM was observed. Achieving the higher relaxivity per NP shown here could enable NP tracking and tumor imaging in combination with future cancer treatments.