In this work, we present a systematic study of MnxFe3-xO4 MNPs (x = 0.14 -1.40) with similar sizes and shapes, synthesized by a one-step thermal decomposition method in order to correlate the Mn 2+ content with their structural, magnetic, heating and relaxivity properties. To this aim, seven MNPs were prepared by varying critical parameters such as the metallic precursors ratio, the final volume of solvent and the ratio of surfactant/metallic precursors. The control of size and morphology allowed us to explore the resulting magnetic anisotropies and the variable heating and relaxivity response in relation to the Mn 2+ content. The impact of polymer coating on the final compositions of MnxFe3-xO4 MNPs and thus, its influence in the crystal structure and magnetic properties was also studied. This work provides important insights into the fundamental understanding of the relationship between Mn 2+ content and their final biomedical application of MnxFe3-xO4 MNPs, paving the way for developing improved therapeutic and imaging agents for various biomedical applications.