The rapid progress in antiferromagnetic and altermagnetic spintronics has led to increased interest in magnetic materials with vanishing net magnetization but strong spin-dependent transport properties. As thin films of such materials become central to device concepts, precise magnetic characterization is essential, to quantify intrinsic moments and interpret transport signatures such as the anomalous Hall effect. In this work, we show that commercial $\mathrm{Mg}\mathrm{O}$ substrates, commonly used in epitaxial growth, often produce substantial parasitic magnetic signals that can match or exceed the response of weakly magnetic films. We identify two major components: a low-field ferromagnetic-like contribution originating from the epi-ready surface, and a temperature-dependent paramagnetic background associated with dilute bulk impurities. These artifacts vary between samples and cannot be corrected using standard linear background subtraction or a measured reference substrate. To address this, we develop and put forward a compensation scheme that combines two complementary, nondestructive measurement protocols. We demonstrate up to 97% efficacy without requiring prior measurements of the bare substrate. The proposed framework enables reliable extraction of intrinsic magnetic signals and provides a general strategy for high-fidelity magnetometry in weakly magnetic thin-film systems, including emerging classes of materials such as topological phases and two-dimensional magnets. We also report the diamagnetic susceptibility of crystalline $\mathrm{Mg}\mathrm{O}$, ${\chi }_{\mathrm{Mg}\mathrm{O}}=-4.0\times {10}^{-7}$ emu/g/Oe.

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• Diamagnetism
• Magnetometry
• Magnetotransport
• Spintronics
• Altermagnets
• Antiferromagnets
• Magnetic thin films
• Topological materials