Nuclear Magnetic Resonance (NMR) has been widely applied to biological tissues. Portable Nuclear Magnetic Resonance (pNMR) has previously been demonstrated to be suitable for the quantification of Mammographic Density (MD), a known risk factor associated with the development of breast cancer (BC). Here we investigate the precision and accuracy of pNMR measurements of MD ex vivo as compared to the gold standards. In this study, ninety breast tissue explants from nine prophylactic mastectomy patients were investigated. The relative tissue water content of the explants was measured using pNMR (MObile Universal Surface Explorer; NMR_MOUSE®) scanner taken as the MD-equivalent quantity. The results were then assessed for precision and accuracy against the gold standards. In each sample, the water content was measured using some combination of three pNMR techniques (apparent T2, diffusion, and T1 measurements) and two gold-standard techniques (micro-CT and H&E histology). Pairwise correlation plots and Bland-Altman analysis were used to quantify the degree of agreement between pNMR techniques and the gold standards. Relative water content measured from apparent T2 relaxation spectra and diffusion decays demonstrated a strong correlation with the H&E and micro-CT results. Bland-Altman analysis yielded bias values -0.4, -2.6, 2.6 and 2.8 water percentage points (pp) and 95% confidence intervals (CI) of 13.1, 7.5, 11.2 and 11.8 pp for the H&E-T2, micro-CT-T2, H&E-diffusion and micro-CT-diffusion comparison pairs, respectively. T1-based measurements, however, were found to be less reliable, with the Bland-Altman limits of agreement of 27.7 and 33.0 pp when compared with H&E and micro-CT, respectively. Apparent T2- and diffusion-based pNMR measurements enable quantification of MD in breast tissue explants with the precision ~10 pp and accuracy ~3 pp or better, making pNMR a promising measurement modality for radiation-free quantification of MD. The current study has successfully estimated the amplitude of the breast components based on the water/fat ratio. The measurement techniques demonstrated their ability to measure the MD of breast tissue in a laboratory study with accuracy, reproducibility, and reliability. Portable NMR provides quantitative tissue composition information while offering the benefits of low cost, portability and flexibility, and operational simplicity.