This study critically analyzed the implementation of the Competency-Based Curriculum (CBC) in Uganda's lower secondary education system, focusing on teacher preparedness, resource adequacy, and assessment alignment as key determinants of implementation success. Employing a mixed-methods research design, the study collected data from 385 teachers, 60 head teachers, 30 curriculum developers and education officers, and 800 students across 45 randomly selected secondary schools in five districts representing urban, peri-urban, and rural settings. Sample size determination using G*Power software with 80% statistical power, 0.05 alpha level, and 0.30 effect size ensured adequate statistical rigor. Data were collected through structured questionnaires, semi-structured interviews, classroom observations, and document analysis, with quantitative data analyzed using descriptive statistics, Pearson correlations, independent samples t-tests, ANOVA, multiple linear regression, and chi-square tests in SPSS version 26, while qualitative data underwent thematic analysis using NVivo software. The findings revealed that teacher preparedness remained at moderate to low levels, with overall mean scores of 2.54 (SD=0.72) on a 5-point scale, and only 48.6% of teachers having received CBC training. A strong positive correlation (r=0.547, p