Countries with fragile health systems like South Sudan experienced significant impacts on routine immunization during the COVID-19 pandemic. Routine immunization in children aged younger than 1 year declined due to pandemic-related constraints and was compounded by the introduction of the COVID-19 vaccine, which was met with hesitancy and reluctance. When South Sudan reported the first COVID-19 case in March 2020, the CORE Group Partners Project (CGPP) rapidly integrated the COVID-19 outbreak response into its ongoing polio eradication activities, leveraging the existing polio infrastructure and human resources. We describe the integration process, results, and challenges and detail the impact of the integration on coverage for both routine immunization and COVID-19 vaccinations. Efforts to integrate COVID-19 vaccination and routine immunization service delivery were implemented in 5 phases: assessing the need, developing multisector collaborations, developing a service delivery plan, assessing implementation readiness, and implementing and evaluating the service delivery plan. Integration efforts prioritized coordination, training vaccinators and volunteers, development of microplans, data management, and last-mile vaccine delivery. Integrated service delivery was implemented through "one-stop shop" sessions where communities accessed routine immunizations for children, COVID-19 vaccinations for adults, and other primary health services. Integrating health service delivery contributed to improved routine immunization coverage among children, improved COVID-19 vaccination coverage among adults, reduced cost for service delivery, and increased access to more comprehensive health services in hard-to-reach communities. COVID-19 vaccinations were delivered at US$4.70 per dose, a cost substantially lower than other reported delivery mechanisms. Integration can yield positive results and improve access to vaccination and other health services for communities. However, it requires clear policy guidelines, commitment, and strong collaboration. Challenges included resistance from stakeholders, overstretched human resources, and diversion of funding and attention from program areas, which were overcome through deliberate high-level advocacy, partnership, and intensified community engagement.