Prevention of mother-to-child transmission of HIV (PMTCT) is a foundational component of a comprehensive HIV treatment program. In addition to preventing vertical transmission to children, PMTCT is an important catch-point for universal test-and-treat strategies that can reduce community viral load and slow the epidemic. However, systematic reviews suggest that care engagement in PMTCT programs is sub-optimal. This study enrolled a cohort of 200 women initiating PMTCT in Kilimanjaro, Tanzania, and followed them to assess HIV care engagement and associated factors. Six months after delivery, 42/200 (21%) of participants were identified as having poor care engagement, defined as HIV RNA >200 copies/mL or, if viral load was unavailable, being lost-to-follow-up in the clinical records or self-reporting being out of care. In a multivariable risk factor analysis, younger women were more likely to have poor postpartum care engagement; with each year of age, women were 7% less likely to have poor care engagement (aRR: 0.93; 95% CI: 0.89, 0.98). Additionally, women who had told at least one person about their HIV status were 47% less likely to have poor care engagement (aRR: .53; 95% CI: 0.29, 0.97). Among women who entered antenatal care with an established HIV diagnosis, those who were pregnant for the first time had increased risk of poor care engagement (aRR 4.16; 95% CI 1.53, 11.28). The findings suggest that care engagement remains a concern in PMTCT programs, and must be addressed to realize the goals of PMTCT. Comprehensive counseling on HIV disclosure, along with community-based stigma reduction programs to provide a supportive environment for people living with HIV, are crucial to address barriers to care engagement and support long-term treatment. Women presenting to antenatal care with an established HIV status require support for care engagement during the crucial period surrounding childbirth, particularly those pregnant for the first time.