This study critically examined Uganda's Universal Primary Education (UPE) policy after 28 years of implementation, investigating the extent to which its performance, quality, and outcomes aligned with evolving transitional expectations for 21st-century competencies in the context of Uganda's aspired knowledge economy and Vision 2040 development goals. Employing a mixed-methods research design, the study collected data from 480 respondents across 12 purposively selected districts representing Uganda's diverse geographical regions, utilizing structured questionnaires, semi-structured interviews, focus group discussions, and documentary analysis to capture multiple stakeholder perspectives including teachers, parents, learners, head teachers, education officials, and employers. Quantitative data were analyzed using univariate descriptive statistics, bivariate tests (chi-square, Pearson correlations, ANOVA, t-tests), and Structural Equation Modeling to examine complex relationships between implementation factors and educational outcomes, while qualitative data underwent thematic content analysis to provide contextual explanations and stakeholder insights. The findings revealed a critical paradox: while UPE had successfully expanded access and achieved gender parity, 71.46% of learners performed at below basic or basic literacy levels, 72.51% demonstrated similar deficiencies in numeracy, and 51.04% exhibited low 21st-century skills, indicating profound quality deficits that undermined the policy's transformative potential. Systemic implementation challenges exerted substantial negative effects on educational outcomes (β = -0.547, p < 0.001), with 39.17% of schools operating with pupil-teacher ratios exceeding 100:1, 68.75% facing critical textbook shortages, 55.63% suffering from poor infrastructure conditions, and 60.21% of teachers reporting low motivation levels. The Structural Equation Modeling demonstrated that teacher quality (β = 0.486), resource allocation (β = 0.423), infrastructure adequacy (β = 0.368), and governance mechanisms (β = 0.312) were significant determinants of educational outcomes, collectively explaining 53.6% of variance in learning achievements. Significant regional disparities persisted, with the Northern Region (M = 1.76) substantially underperforming the Central Region (M = 2.87, p < 0.001), while urban-rural infrastructure gaps (Cohen's d = 0.59) evidenced the emergence of a two-tier education system that perpetuated inequalities. The strong correlation between 21st-century skills and employment readiness (r = 0.689, p < 0.001) underscored the widening gap between UPE curriculum delivery and labor market demands, threatening graduates' competitiveness and national development aspirations. Stakeholder satisfaction was overwhelmingly negative, with 53.33% perceiving UPE as very ineffective (mean = 2.28), reflecting a crisis of confidence in the policy's capacity to deliver meaningful educational outcomes despite nearly three decades of implementation and substantial public investment. The study concluded that UPE had created an illusion of educational progress through enrollment statistics while systematically failing to establish quality foundations necessary for individual advancement and national development. Key recommendations included comprehensive teacher development and welfare reforms to address qualification gaps and motivation deficits; equity-focused resource allocation strategies prioritizing marginalized regions and rural areas to reduce disparities; and curriculum reforms emphasizing competency-based learning aligned with 21st-century skills requirements, accompanied by strengthened governance and accountability mechanisms. These evidence-based interventions were identified as critical for transitioning UPE from mere access provision to genuine quality delivery capable of preparing learners for meaningful participation in Uganda's social, economic, and civic life, thereby fulfilling the policy's long-deferred transformative promise at this critical 28-year juncture.