Background Given the support for the numerous benefits of breastfeeding, a better understanding of social and health determinants is necessary, particularly in under-researched populations. We examined determinants of breastfeeding initiation and duration using a national cohort of Chilean mothers. Methods Participants included 13 738 families enrolled in the Encuesta Longitudinal de la Primera Infancia cohort. Data were collected in 2010 and 2012. Families from all regions of the country were considered. Breastfeeding information was collected via maternal report and standardised assessments were used to collect information on maternal IQ and personality. Logistic and linear regressions were used to identify predictors of breastfeeding initiation and duration. Results Breastfeeding was initiated by 95.2% of mothers. Variation in duration of breastfeeding was large, ranging from 1 to 48 months (M = 11.74; SD = 8.74). Maternal IQ, low-risk prenatal behaviours, conditions at birth and the presence of a partner were relevant predictors of both initiation and duration of breastfeeding, whereas personality and contextual/socioeconomic factors were relevant only for breastfeeding duration. Differences between regions were observed. Rates of caesarean deliveries are alarmingly high and triple that of the global WHO recommendations, at 45% of deliveries in Chile, which are implicated in both initiation and duration. Conclusions Breastfeeding rates exceed Chilean target goals although vary by region. Global targets now need to be focused on. Social and health determinants are implicated in both initiation and duration of breastfeeding. These findings suggest important targets for policy development and breastfeeding initiatives in Chile, particularly concerning the reduction of surgical deliveries.