Breastfeeding peer counseling support programs for low-income women have been implemented across the United States. Data from one such program were used to examine participant and program characteristics, of those enrolled prenatally (n = 2168) or postnatally (n = 2899), and to determine how these characteristics affected breastfeeding outcomes. Shorter breastfeeding duration was significantly predicted by introduction of formula on day 1 postpartum in participants enrolled prenatally (-37.9 days [95% CI: -57.9 to -17.9]) as well as postnatally (-49.1 days [95% CI: -63.4 to -34.8]). In both groups, increasing maternal age and previous breastfeeding experience were associated with significantly longer breastfeeding duration. Breastfeeding duration may be improved in programs by targeting younger mothers, those without breastfeeding experience, and focusing on delaying the introduction of formula.