Background The socioeconomic burden of early onset dementia (EOD) defined as disease onset before the age of 65 years, is substantial due to its widespread disabling effects in relatively young individuals. While dementia is widely recognised as a major contributor to mortality among the elderly, only a limited number of studies have assessed survival and factors associated with prognosis specifically in EOD. Methods A population-based cohort study, encompassing all incident EOD cases from two defined regions in Finland. The survival and all-cause mortality rates in EOD and its subtypes were evaluated from January 2010 to December 2021. All visits at the dementia outpatient clinics were reviewed and manually re-assessed (n=12 490), resulting in 794 validated EOD cases of Alzheimer’s disease (AD), frontotemporal dementia (FTD), alpha-synucleinopathy (α-SYNU) and other EOD spectra. Region-, age- and sex-matched control groups without neurodegenerative diseases were created from nonselective general population data registers (1:10 case to control ratio, 7930 controls in total). Results The median survival for EOD was 8.7 years, with the shortest survival in the FTD (6.9 years) and α-SYNU groups (7.0 years), followed by the AD group (9.9 years). Compared with controls, mortality was significantly higher in the total EOD group (HR=6.56, 95% CI=5.56–7.74, p<0.001). Among the dementia subtypes, FTD spectrum patients had the highest all-cause mortality risk compared with controls (HR=13.75, 95% CI=10.25–18.43, p<0.001). Male sex, older age, several comorbidities and lower level of education were associated with increased mortality, but these were not EOD-specific. Conclusion EOD diagnosis significantly deteriorates patients’ survival, with significant variation between different diagnostic groups and in relation to patients’ demographic factors.