This study investigated the relationship between head teacher instructional leadership and Competency-Based Curriculum (CBC) implementation outcomes at the school level in Uganda. A cross-sectional quantitative design was adopted, drawing a sample of 260 schools through stratified random sampling across government and private, and rural and urban strata. Head teacher instructional leadership was measured across three dimensions adapted from the Principal Instructional Management Rating Scale (PIMRS) defining the school mission, managing the instructional program, and promoting a positive school learning climate while CBC implementation outcomes were captured through a composite index reflecting curriculum delivery fidelity, continuous assessment practice, resource utilization, and integration of generic skills. Data were analyzed using univariate, bivariate, and multivariate statistical techniques, complemented by sensitivity analyses. The mean CBC implementation outcome score was 56.3 (SD = 10.4) out of 100, and the mean head teacher instructional leadership composite score was 58.1 (SD = 10.5). All three instructional leadership dimensions were significantly and positively correlated with CBC implementation outcomes (r ranging from 0.43 to 0.51, p < 0.001), and outcomes differed significantly by school type and location. In the multivariate ordinary least squares regression, all three instructional leadership dimensions, the instructional resource index, in- service training hours, school type, and location remained statistically significant independent predictors, with the full model explaining 47.7% of the variance in CBC implementation outcomes (R² = 0.477, F = 27.58, p < 0.001). Sensitivity analyses — including a reduced composite-score model, a logistic regression on a binarised high-fidelity implementation outcome, and non-parametric bootstrap resampling with 1,000 replications — confirmed the stability and robustness of the observed leadership-outcome relationship. The findings affirm that head teacher instructional leadership is a central, modifiable driver of CBC implementation success at the school level, and the study concludes with recommendations for targeted leadership capacity building, equitable resource investment, and routine school-