BACKGROUND: Some children born to adolescent mothers may have developmental challenges, while others do not. Research focusing on which children of adolescent mothers are at the highest risk for cognitive delay is still required. Both maternal HIV status and maternal mental health may affect child development. An examination of maternal mental health, especially in the presence of maternal HIV infection may be timely. This study explores the relationship between the mental health of adolescent mothers (comparing those living with and not living with HIV) and the cognitive development performance scores of their children. Additional possible risk and protective factors for poor child development are explored to identify those children born to adolescent mothers who may be at the greatest risk of poor cognitive development. METHODS: Cross-sectional data utilised within the analyses was drawn from a large cohort of adolescent mothers and their children residing in South Africa. Detailed study questionnaires were completed by adolescent mothers relating to their self and their child and, standardised cognitive assessments were completed by trained researchers for all children using in the Mullen Scales of Early Learning. Chi-square, t-tests (Kruskal Wallis tests, where appropriate), and ANOVA were used to explore sample characteristics and child cognitive development scores by maternal mental health status (operationalised as likely common mental disorder) and combined maternal mental health and HIV status. Multivariable linear regression models were used to explore the relationship between possible risk factors (including poor maternal mental health and HIV) and, child cognitive development scores. RESULTS: The study included 954 adolescent mothers; 24.1% (230/954) were living with HIV, 12.6% (120/954) were classified as experiencing likely common mental disorder. After adjusting for covariates, maternal HIV was found to be associated with reduced child gross motor scores (B = -2.90 [95%CI: -5.35, -0.44], p = 0.02), however, no other associations were identified between maternal likely common mental disorder, or maternal HIV status (including interaction terms), and child cognitive development scores. Sensitivity analyses exploring individual maternal mental health scales identified higher posttraumatic stress symptomology scores as being associated with lower child cognitive development scores. Sensitivity analyses exploring potential risk and protective factors for child cognitive development also identified increased maternal educational attainment as being protective of child development scores, and increased child age as a risk factor for lower development scores. CONCLUSIONS: This study addresses a critical evidence gap relating to the understanding of possible risk factors for the cognitive development of children born to adolescent mothers affected by HIV. This group of mothers experience a complex combination of risk factors, including HIV, likely common mental disorder, and structural challenges such as educational interruption. Targeting interventions to support the cognitive development of children of adolescent mothers most at risk may be of benefit. Clearly a basket of interventions needs to be considered, such as the integration of mental health provision within existing services, identifying multiple syndemics of risk, and addressing educational and structural challenges, all of which may boost positive outcomes for both the mother and the child.