Ensuring the uninterrupted availability of life-saving Reproductive, Maternal, Newborn, and Child Health (RMNCH) commodities at the primary health care (PHC) level is vital for advancing maternal and child health outcomes in low- and middle-income countries. Although many health programs emphasize routine stock monitoring to strengthen supply chains, longitudinal data on facility-level stock levels remain scarce. This study assessed the monthly availability of essential RMNCH commodities across PHCs in Katsina State, Nigeria, over a 12-month period. A longitudinal stock status assessment was conducted from January to December 2018 across an average of 198 PHCs. Fourteen tracer commodities, including three vaccines, two contraceptive methods, and nine therapeutic agents, were assessed monthly. Eleven trained data collectors reviewed stock registers and conducted physical verification of commodities. Stock levels were evaluated against facility-specific minimum thresholds derived from average monthly consumption, with compliance defined as maintaining stock at or above the threshold for three consecutive months. Vaccines such as Bacille Calmette–Guérin, pentavalent, and tetanus–diphtheria maintained near-universal availability (≥99%) throughout the year. Conversely, several therapeutic commodities showed suboptimal availability: sulfadoxine–pyrimethamine (27%), amoxicillin dispersible tablets (34%), and folic acid (47%). Moderate availability was observed for misoprostol (60%) and zinc and oral rehydration salts (52%). Contraceptive availability was comparatively high, averaging 29% for oral pills and 91% injectables. These findings reveal strong vaccine supply performance but persistent stockouts of essential maternal and child health commodities, underscoring the urgent need for integrated supply chain reforms to ensure consistent availability of all RMNCH inputs at the point of care.