Background/Objectives: Pakistan has one of the highest burdens of adolescent anaemia in South Asia. A Weekly Iron-Folic Acid Supplementation (WIFAS) program for school-going adolescent girls aged 10–19 years was piloted in the Lodhran District, Punjab, Pakistan. Methods: A quantitative cross-sectional study was conducted among 750 girls, selected by multi-stage random cluster sampling from 50 public schools across Lodhran; qualitative data were generated through in-depth interviews and focus group discussions with 327 stakeholders. Quantitative data were analyzed using a chi-square test in SPSS. Qualitative data were analyzed in NVIVO using thematic analysis based on the Socio-Ecological Model, enabling environment and institutional systems, community engagement, behavior change and adolescent empowerment and agency. Coding was guided by overarching research questions of acceptability, feasibility and impact. Cases of anaemia averted were estimated using program data and outcome modeling. Qualitative findings were triangulated with quantitative results. Results: The program reached 30,506 girls and was estimated to have averted 5659 cases of anaemia. WIFAS coverage increased from 0% to 100%, with a high level of adherence (80%). Knowledge about anaemia, its signs/symptoms and preventative measures increased significantly at endline. Dietary practices showed positive changes, with significant increase in girls never skipping a meal. Stakeholders attributed success to the creation of an enabling environment through institutional alignment and multisectoral collaboration, culturally adapted, adolescent-friendly materials, and robust community mobilization that proactively addressed socio-cultural barriers. Conclusions: The school-based WIFAS program for reducing anaemia among adolescent girls was feasible and acceptable in a context where few nutrition services are available to girls.