The timely onset of stage II lactogenesis (OL) is important for successful breastfeeding and newborn health. Several risk factors for delayed OL are common in women with a history of gestational diabetes mellitus (GDM), which may affect their chances for successful breastfeeding outcomes. We investigated the prevalence and risk factors associated with delayed OL in a racially and ethnically diverse cohort of postpartum women with recent GDM. We analyzed data collected in the Study of Women, Infant Feeding and Type 2 Diabetes After GDM Pregnancy (SWIFT), which is a prospective cohort of women diagnosed with GDM who delivered at Kaiser Permanente Northern California hospitals from 2008 to 2011. At 6-9 wk postpartum, delayed OL was assessed by maternal report of breast fullness and defined as occurring after 72 h postpartum. We obtained data on prenatal course and postdelivery infant feeding practices from electronic medical records and in-person surveys. We used multivariable logistic regression models to estimate associations of delayed OL with prenatal, delivery, and postnatal characteristics. The analysis included 883 SWIFT participants who initiated breastfeeding and did not have diabetes at 6-9 wk postpartum. Delayed OL was reported by 33% of women and was associated with prepregnancy obesity (OR: 1.56; 95% CI: 1.07, 2.29), older maternal age (OR: 1.05; 95% CI: 1.01, 1.08), insulin GDM treatment (OR: 3.11; 95% CI: 1.37, 7.05), and suboptimal in-hospital breastfeeding (OR: 1.65; 95% CI: 1.20, 2.26). A higher gestational age was associated with decreased odds of delayed OL but only in multiparous mothers (OR: 0.79; 95% CI: 0.67, 0.94). One-third of women with recent GDM experienced delayed OL. Maternal obesity, insulin treatment, and suboptimal in-hospital breastfeeding were key risk factors for delayed OL. Early breastfeeding support for GDM women with these risk factors may be needed to ensure successful lactation. This trial was registered at clinicaltrials.gov as NCT01967030.