Paraprofessional home visitors trained to improve multiple outcomes (HIV, alcohol, infant health and malnutrition) have been shown to benefit mothers and children over 18 months in a cluster randomised controlled trial. These longitudinal analyses examine the mechanisms which influence child outcomes at 18 months post-birth in Cape Town, South Africa. The results were evaluated using structural equation modelling, specifically examining the mediating effects of prior maternal behaviours and a home visiting intervention post-birth. Twelve matched pairs of neighbourhoods were randomised within pairs to: (1) the control condition, receiving comprehensive health care at community primary health care clinics (n = 12 neighbourhoods; n = 594 pregnant women), or (2) the Philani Intervention Program, which provided home visits by trained, paraprofessional community health workers, here called Mentor Mothers, in addition to clinic care (n = 12 neighbourhoods; n = 644 pregnant women). Recruitment of all pregnant neighbourhood women was high (98%) with 88% reassessed at 6 months and 84% at 18 months. Infants’ growth and diarrhoea episodes were examined at 18 months in response to the intervention condition, breastfeeding, alcohol use, social support and low birth weight, controlling for HIV status and previous history of risk. We found that randomisation to the intervention was associated with a significantly lower number of recent diarrhoea episodes and increased rates and duration of breastfeeding. Across both the intervention and control conditions, mothers who used alcohol during pregnancy and had low birth weight infants were significantly less likely to have infants with normal growth patterns, whereas social support was associated with better growth. HIV infection was significantly associated with poor growth and less breastfeeding. Women with more risk factors had significantly smaller social support networks. The relationships among initial and sustained maternal risk behaviours and the buffering impact of home visits and social support are demonstrated in these analyses.