Tuberculosis (TB) remains a major global public health concern, particularly in low- and middle-income countries, where delayed diagnosis and treatment continue to contribute to ongoing transmission, poor treatment outcomes, and increased mortality. Patient and health system delays remain critical barriers to effective TB control. This study assessed the determinants and consequences of patient and health system delays in tuberculosis diagnosis and treatment among individuals aged 15 years and above at Kenyatta National Hospital, Nairobi, Kenya. Methods: A retrospective cohort study was conducted among 127 tuberculosis patients aged ≥15 years receiving treatment at Kenyatta National Hospital. Data were collected using structured questionnaires and clinical records. Descriptive statistics were used to summarize participant characteristics, while chi-square tests and logistic regression analyses were performed to determine factors associated with delay. Results were presented as adjusted odds ratios (AORs) with 95% confidence intervals. Results: The majority of participants were aged 40–49 years (32.3%). More than half of the participants (59.8%) experienced delays exceeding two months before seeking healthcare services. Perceived stigma (96.9%), fear following diagnosis (66.9%), and long distance to health facilities were major barriers to timely care-seeking. Participants who initially sought care from informal providers and practiced self-medication experienced longer delays before diagnosis and treatment initiation. Age group, distance to health facility, and education level were significantly associated with delay at bivariate analysis (p < 0.05), although no independent predictors remained statistically significant after multivariable analysis. Conclusion: Patient and health system delays remain major challenges in tuberculosis control. Stigma, fear, and barriers to healthcare access contribute substantially to delayed diagnosis and treatment. Strengthening comm