Children are often only identified with a developmental delay when they enter primary school due to developmental delays hindering academic progress. Detection of at-risk children in low-income communities is typically unavailable due to several challenges. This study validated an mHealth-based developmental screening tool as a potential time- and cost-effective way of delivering services for preschool children. This cross-sectional within-subject study screened 276 preschool children from low-income communities using the mHealth Parents’ Evaluation of Developmental Status (PEDS) tools. The mHealth PEDS tools’ performance was then evaluated by comparing caregiver concerns according to the PEDS tools with results obtained using a reference standard assessment tool, the Vineland-3. The mHealth PEDS tools identified 237 (85.9%) of children at risk of developmental delay compared to 80.1% (n = 221) of children identified with a developmental delay using the Vineland-3. Sensitivity of the PEDS tools was high (92.6%) with low specificity (22.5%) using US standardised criteria. Literacy skills were found to be most delayed, according to the PEDS: DM (89.3%; n = 142) and Vineland-3 (87.1%; n = 134). Low specificity of the prescribed criteria may require the implementation of adapted referral criteria within low socio-economic status (SES) settings. The mHealth PEDS tools may still be valuable for preschool developmental surveillance of children within low SES settings. It is recommended, however, that children who are identified with a developmental concern undergo a second screen to reduce false positives.