Background: Young-onset dementia (YOD) is associated with substantial social and clinical burden and an increased risk of premature mortality, yet its determinants have not been systematically synthesised. This review aimed to identify and evaluate factors associated with mortality in YOD within a biopsychosocial framework. Methods: A systematic review was conducted in accordance with PRISMA guidelines. MEDLINE, Embase, PsycINFO, CINAHL, Web of Science, and DOAJ were searched from inception to 14 May 2025. Quantitative studies examining mortality or survival outcomes in YOD were included. Study quality was assessed using the National Heart, Lung, and Blood Institute tool. Due to heterogeneity, findings were synthesised narratively within a biopsychosocial framework. Results: Sixteen studies (1986–2024) were included. Most were observational cohort studies with sample sizes ranging from 71 to 239,602 and follow-up durations of 4–18 years. Mortality was most consistently associated with biological fixed factors, particularly increasing age and male sex. Evidence for tractable biological factors, including clinical symptoms and comorbidities, was mixed, although diabetes mellitus showed relatively consistent associations with increased mortality. Psychosocial determinants were infrequently examined, with limited and inconsistent evidence. Health and social service factors were rarely investigated. Most determinants were examined in single studies, limiting quantitative synthesis. Conclusions: Mortality in YOD is largely driven by non-modifiable factors, with limited evidence on modifiable determinants. More rigorous prospective studies are needed to inform targeted interventions.