Introduction: To evaluate scalp hair steroid concentrations as a monitoring tool for androgen control and metabolic outcomes in adults with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency. Methods: 17-hydroxyprogesterone (17-OHP), androstenedione, testosterone, cortisol, cortisone, progesterone, prednisolone, and prednisone concentrations were measured in scalp hair by means of LC-MS/MS in 27 women and 15 men with CAH and controls (37 women, 42 men). Results: In CAH men and women, 17-OHP levels in hair showed a significant positive correlation with corresponding levels in serum (ρ = 0.654; p = 0.01; ρ = 0.553, p = 0.003 respectively), while total testosterone levels were only significantly correlated in CAH men (ρ = 0.543; p = 0.036). Androstenedione levels did not show a significant correlation. Receiver-operating characteristic (ROC) curve analysis indicated that a cutoff value of 21.7 pg/mg for 17-OHP in hair provided a sensitivity of 100% and a specificity of 88.9% for identifying men with elevated serum androstenedione. Hair 17-OHP in women showed a poorer performance in terms of identifying those with elevated androstenedione serum levels. However, when applying a cutoff value of 5.5 for the free androgen index as a marker of significant hyperandrogenism in CAH women, 17-OHP >27.6 pg/mg in hair provided a sensitivity of 100% and a specificity of 95.8% (AUC 0.986, 95% CI 0.945–1.000; p < 0.001). Neither hair cortisol nor markers of adrenal androgen control in hair showed significant associations with cardiometabolic outcome or bone health. Conclusion: This study shows that scalp hair 17-OHP concentrations may be a promising noninvasive long-term parameter for treatment monitoring in adult patients with CAH.