Adequate and quality immunization coverage plays a key role in controlling the outbreaks of vaccine preventable diseases. Places where immunization coverage is low, vaccine preventable diseases contribute to worse health outcomes. This is especially true in Indian slum dwellings where 33.0% of the urban population live. The aim of the study was to explore the coverage, quality, and correlates of primary immunization under national immunization program among children aged 12–23 months, living in slums of Mumbai. A community based cross-sectional survey was conducted. Parents or caretakers of 550 eligible children aged 12–23 months were interviewed using a structured interview schedule. Regression analysis was used to detect correlates of full immunization coverage (children who received one dose each of BCG, measles, and three doses each of DPT, OPV, and HBV by his/her first birthday) and of quality immunization coverage (children who received primary vaccines at appropriate age and intervals as mentioned above and had filled immunization card). Out of total 550 children, 402 (73.1%), 131 (23.8%), and 17 (3.1%) were fully, partially, and unimmunized, respectively. Almost 86.0% children received quality immunization coverage. In the regression analysis, reminder for immunization services was found to be the single most significant correlate of full and quality immunization coverage.In this study, full immunization coverage was found to be below the expected level. This study also revealed that the awareness regarding the importance of adequate immunization was still lacking in the slum population. Emphasizing on reminders for immunization services, encouraging institutional deliveries, and scaling up use of postnatal care services may act as keys to improving the immunization coverage in Indian slums.