This article aims to identify factors associated with breastfeeding continuation for at least 12 months among working mothers in a hospital in the south of Brazil. We conducted a cross-sectional study, interviewing 251 women who breastfed after returning to work. Eligibility criteria included non-twin biological children aged between 12 and 36 months and the absence of an illness (mother and/or child) that could affect breastfeeding. The association between breastfeeding continuation and the exposure variables was tested using Poisson multivariate regression. Only one work-related variable showed a significant association with the outcome. Working only during the day increased the prevalence of BF continuation for at least 12 months by 37%. The following non-work-related factors showed a positive association with the outcome: mothers without a college degree; mothers with at least 12 months’ prior breastfeeding experience; child not given milks other than breast milk when the mother returned to work, and not using a pacifier. The following variables showed a negative association with the outcome: older maternal age; older gestational age; mother receiving support from the child’s caregiver; and mother receiving professional breastfeeding support. Non-work-related factors had a greater influence on breastfeeding continuation for at least 12 months among working mothers.