Weak accountability hinders the effectiveness of routine immunization (RI) systems in low- and middle-income countries, yet studies on accountability of immunization programs are scarce. Aim: The study explored stakeholders' perspectives on the functioning of internal accountability within the National Program on Immunization in Enugu State, southeast Nigeria. We used semi-structured in-depth interviews to collect data from RI officials at state government, local government, and health facility levels (n = 35) in Enugu State between June and July 2021. We adopted maximum variation sampling to purposively select individuals with roles in immunization. The interview guide was developed based on an accountability framework with three dimensions-the axes of power, ability, and justice. Data were analyzed thematically using NVivo software (version 11). The major themes were role clarity, performance standards, supervision, data use, human resources, funding, motivation, sanctions, political influence, and community engagement. Performance targets for immunization coverage and reporting timeline were not always met due to multiple accountability failures. Weaknesses in the formal rules that distribute roles among the immunization workforce comprise a lack of deployment letters, unavailability of job descriptions, and inadequate staff orientation. Local officials have a narrow decision space regarding staff posting, transfer, and discipline. Performance accountability was constrained by staff shortages, uneven staff distribution, absenteeism, infrequent supervision, weak data monitoring system, and underfunding. Despite being motivated by job recognition and accomplishments, low motivation from an insecure working environment and lack of financial incentives undermined the constructive agency of service delivery actors. The sanctions framework exists but is weakly enforced due to fear of victimization. Political commitment to the immunization program was low. Yet, political decision-makers interfered with staff recruitment, distribution, and discipline. Community engagement improved resource availability through paid volunteer health workers and maintenance of facilities. However, health facility committees were poorly resourced, non-functional, and lacked the power to sanction erring health workers. Immunization service delivery actors can be held accountable for program performance when there are sufficient formal instruments that provide roles and responsibilities, needed resources, motivated and supervised staff, an effective sanctions framework, genuine political participation, and strong community engagement.