The benefits of initiating breastfeeding in the first hour of life and exclusively breastfeeding thereafter are well documented. However, little is known about how best to promote these practices. In this study, we assess the impact of Buddhist nuns and wat (pagoda) grannies on optimal breastfeeding behaviours in rural Cambodia. We did so by interviewing randomly selected mothers of infants less than 6 months of age. A total of 440 mothers in programme and control communities completed the baseline survey (before programme start-up), and 467 mothers responded 1 year later. Mothers’ exposure to breastfeeding promotion activities was high. At follow-up, 76% of mothers in programme communities indicated that nuns had advised them about breastfeeding, 73% received a home visit and 72% reported attending an educational session. At baseline, mothers in programme communities were 11% more likely (RR = 1.11, CI: 0.74–1.68) than mothers in control communities to initiate breastfeeding in the first hour of life. At follow-up, they were 62% more likely (RR = 1.62, CI: 1.30–2.01) to do so. Similarly, at baseline, mothers in programme communities were 54% more likely (RR = 1.54, CI: 1.21–1.96) than mothers in control communities to breastfeed exclusively in the previous 24 h. At follow-up, they were 81% more likely (RR = 1.81, CI: 1.49–2.21) to do so. Programme planners may consider using community-based volunteers as one strategy to improve breastfeeding practices and child survival.