The Adolescent Health Quality of Care (AHQOC) index is a tool designed to evaluate the quality of facility-based adolescent sexual and reproductive health (ASRH) services. This descriptive cross-sectional study aimed to validate the AHQOC index in 27 primary and secondary public health facilities located in a rural and an urban local government area (LGA) of Ogun State, Nigeria. To conduct the study, 12 mystery clients (MCs) were recruited and performed 144 visits to the health facilities. The MCs were young males and females who were seeking information on premarital sex, pregnancy prevention, sexually transmitted infections (STIs), and contraception. The validity, and reliability of the AHQOC index were evaluated using exploratory factor analysis, Cronbach’s Alpha, and intra-class correlation coefficient tests. The Kaiser-Meyer-Olkin test result for the initial 37-item pool was 0.7169, and the final tool retained 27 items with a Cronbach’s Alpha of 0.80. Two subscales of the index had Cronbach’s Alpha of 0.76 and 0.85. The intra-rater consistency assessed by the intra-class correlation coefficient was 0.66 (0.10–0.92) p = 0.001 for the urban LGA and 0.72 (0.37–0.91) p = 0.001 for the rural LGA. Positive and statistically significant relationships were observed between the full scales and subscales and the validity item (MC ranking of health worker on proficiency from 1 to 10). The results of this study demonstrate that the validated AHQOC index is a valuable tool for assessing the quality of ASRH services in public health facilities.