Background: Dementia policy states that the early diagnosis of dementia can keep people living well for longer; however, there is little robust evidence to support this. Mild cognitive impairment (MCI) is considered a prodrome to dementia and can aid with the earlier diagnosis of dementia. Objective: The objective of this study was to use a previous diagnosis of MCI, before dementia, as a proxy for early diagnosis to investigate the relationship between an early diagnosis and mortality. Methods: A retrospective cohort study of electronic health care records from South London and Maudsley NHS. Patients aged 50+, diagnosed with dementia between January 2008 and November 2018, were divided into two groups: those with a previous diagnosis of MCI (early diagnosis) and those without. Cox regression models used to compare the risk of mortality between groups. Results: Of 18,557 participants, 5.6%(n = 1,030) had an early diagnosis; they had fewer cognitive, psychiatric, and functional problems at dementia diagnosis. The early diagnosis group had a reduced hazard of mortality (HR = 0.86, CI = 0.77–0.97). However, the magnitude of this effect depended on the scale used to adjust for cognitive difficulties. Conclusion: A previous diagnosis of MCI is a helpful proxy for early diagnosis. There is some evidence that an early diagnosis is associated with a reduced risk of mortality; however, it is not clear how Mini-Mental State Exam scores affect this relationship. While these findings are promising, we cannot be conclusive on the relationship between an early diagnosis and mortality.