Adolescence is a critical period for the emergence of mental health disorders, particularly among adolescent girls and young women (AGYW) in resource-limited settings like sub-Saharan Africa. Despite increasing awareness of mental health challenges in this population, evidence remains limited on the psychometric properties of common tools. This study evaluates the psychometric properties of the PHQ-9, GAD-7, and PC-PTSD-5 scales in South African AGYW to measure depression, anxiety, and post-traumatic stress disorder (PTSD) among 642 South African AGYW (ages 15-19). Results demonstrate that the GAD-7 showed robust psychometric properties, including configural, metric, and scalar invariance across English and IsiXhosa speakers, indicating its reliability across linguistic groups. The PHQ-9 displayed adequate factor structure and construct validity but showed lower item-level scalability for certain items (e.g., "little interest," "poor appetite") in Mokken analysis, suggesting potential cultural or contextual influences on item performance. The PC-PTSD-5 demonstrated challenges with scalar invariance, particularly for the "avoidance" item, indicating potential linguistic or cultural differences in interpretation. Despite this, the scale exhibited acceptable overall scalability. Correlations among scales and with parenting and intimate partner violence provided evidence of construct validity, showing expected relationships between mental health symptoms. These findings highlight the GAD-7's utility as a screening tool for anxiety in this population while underscoring the need for further adaptation of the PHQ-9 and PC-PTSD-5. This study emphasizes the importance of culturally sensitive, linguistically appropriate tools for mental health assessment in diverse settings.