Iron (Fe) is an essential mineral for human health, but chronic overexposure may induce oxidative stress, particularly in adolescents undergoing physiological and endocrine changes. Scalp hair was collected from 97 adolescents (13–16 years old; 68 girls) living in Alcalá de Henares, an urban–industrial municipality near Madrid, and analysed using ICP-MS (LoD = 1.148 µg/g). Fe was detected in 100% of samples and showed clear sex dependency: significantly higher concentrations were recorded in females than in males [median (range), µg/g: 5.524 (3.167–13.262) vs. 4.464 (2.666–6.173); p = 0.000057]. This effect may reflect hormonal differences, as the endocrine system typically becomes active earlier in females. Nevertheless, potential confounders such as dietary iron intake, supplements, or cosmetic hair treatments could also contribute to variability and warrant further consideration. Hair Fe levels were correlated with matched surface soil samples from the city. Fe was detected in 100% of soils, ranging from 12,588 to 54,169 µg/g (median 26,159 µg/g). A modest but significant correlation was observed between hair and soil concentrations (r = 0.21, p < 0.05), stronger in males (r = 0.43, p < 0.01) and females (r = 0.36, p < 0.01). Fe concentrations did not differ significantly across the four residential areas defined in Alcalá (p = 0.370). These results support the value of hair as a non-invasive biomonitoring matrix for Fe exposure and emphasize the importance of sex-disaggregated and geospatial analyses. Establishing age- and sex-specific reference values could facilitate the inclusion of hair biomonitoring in public health surveillance and preventive strategies.