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This article describes the validation of analytical methods for the determination of Manganese (Mn) and lead (Pb) by graphite furnace atomic absorption spectrometry (GFAAS) in human hair and nail samples. Method validation parameters such as linearity, repeatability, reproducibility, and precision were determined. In addition, the limit of detection (LOD), the limit of quantification (LOQ), and measurement uncertainty were calculated. The developed method was linear in the concentration ranges of 0.001–0.015 and 0.002–0.020 µg·L−1 of Mn and Pb, respectively. The determination coefficients obtained were greater than 0.995. The recoveries obtained after the addition of the standard concentration for the metals ranged from 84.80–107.98%, with a precision not exceeding 12.97% relative standard deviation. The calculated LOD and LOQ for Mn and Pb are within the ranges established by Commission Regulation (EU) No. 836/2011. The expanded uncertainty was estimated to be less than 9.93–6.59% for Mn and Pb. Matrix effects were also studied, finding a smooth effect in both matrices. The analysis of 30 samples of each type revealed the presence of Mn in 30 and Pb in 13 samples. Overall, the proposed validation method was considered optimal for the determination of Mn and Pb.