Purpose Kaposi sarcoma (KS) remains one of the most common HIV-associated malignancies in sub-Saharan Africa (SSA). While loss to follow-up (LTFU) has been well documented among KS patients managed within HIV primary care, little is known about retention after patients transition into specialized oncology care, where treatment pathways, toxicities, costs, and follow-up schedules differ substantially. Because LTFU is unlikely to occur at random, patients who disengage from care may differ systematically from those retained with respect to disease severity, treatment response, and mortality risk, potentially biasing survival estimates and underestimating cancer-related mortality. This study aimed to estimate the cumulative incidence of LTFU among patients with KS receiving care at Tanzanias national cancer referral center, accounting for death as a competing event, and to identify factors associated with LTFU. Methods This retrospective cohort study included 251 patients with KS treated at Ocean Road Cancer Institute (ORCI) between January 2021 and December 2023. The primary outcome was LTFU, with death treated as a competing event. Cumulative incidence of LTFU at 6, 12, 18, and 24 months was estimated using the cumulative incidence function. Predictors of LTFU were assessed using univariable and multivariable Fine-Gray subdistribution hazards regression. Results Among 251 patients, 214 (85.3%) had epidemic (HIV-associated) KS and 37 (14.7%) had endemic (non-HIV-associated) KS. Males accounted for 62.2%. Accounting for death as a competing event, the cumulative incidence of LTFU was 27.6% (95% CI, 22.0-33.1) at 6 months, 36.4% (95% CI, 30.5-42.4) at 12 months, 43.3% (95% CI, 37.2-49.4) at 18 months, and 46.6% (95% CI, 40.4-52.7) at 24 months. In multivariable Fine-Gray regression, absence of oral involvement (adjusted subdistribution hazard ratio [aSHR], 0.37), reachable telephone contact (aSHR, 0.54), and initial chemotherapy rather than radiotherapy (aSHR, 0.44) were independently associated with lower risk of LTFU. Conclusion Nearly half of patients with KS were LTFU within two years, substantially limiting reliable assessment of cancer outcomes in this setting. Strengthening retention strategies, including maintaining reliable patient contact information and implementing routine phone-based follow-up, may offer feasible, scalable approaches to improve continuity of care, enhance survival monitoring, and strengthen cancer surveillance in resource-limited settings.