Abstract: Continuity of care is essential for Veterans with serious mental illness (SMI), as lapses in treatment are common and linked to increased mortality. This study evaluated mortality rates and causes of death among 9,013 Veterans with schizophrenia-spectrum or bipolar disorders who were lost to Veterans Health Administration (VHA) care and identified for the SMI Re-Engage program in 2016–2017. Compared to all Veterans receiving VHA care from 2015–2019, Veterans in the SMI Re-Engage cohort had nearly double the mortality risk (standardized mortality ratio [SMR] 1.96, 95% CI: 1.82–2.09). The leading causes of death were heart disease (22.3%) and cancer (14.5%), followed by chronic lower respiratory disease and dementia-related conditions. These findings underscore the importance of primary care continuity and targeted re-engagement efforts to reduce preventable mortality among Veterans with SMI.