Aim: The propensity to breastfeed is a matter of public concern because of the favourable effects for infants. However, very few studies have described the influence of paternal variables upon duration of breastfeeding. The aim of this study was to describe the effects of fathers’ socioeconomic status and their use of paternity leave on breastfeeding duration for infants up to 1 year of age. Methods: A prospective population-based cohort study was undertaken. Data on breastfeeding, registered in databases in two Swedish counties for 1993—2001, were matched with data on socioeconomic status and paternity leave obtained from Statistics Sweden. Fathers of 51,671 infants were identified and included. Results: Infants whose fathers had a lower level of education, were receiving unemployment benefit and/or had a lower equivalent disposable household income were significantly less likely to be breastfed at 2, 4, 6, 9, and 12 months of age. Infants whose fathers did not take paternity leave during the infant’s first year were significantly less likely to be breastfed at 2 (p < 0.001), 4 (p < 0.001), and 6 months (p < 0.001). Conclusions: This paper shows that an enabling of an increased involvement from fathers during the infants’ first year of life, such as by paid paternity leave, may have beneficial effects on breastfeeding up to 6 months of age. A more systematic approach to supporting fathers’ involvement may be particularly valuable to those infants whose fathers have a lower socioeconomic status.