<b><i>Introduction:</i></b> The aim of the study is to describe the frequency of frailty in people with a new diagnosis of mild dementia due to Alzheimer’s disease (AD) and dementia with Lewy bodies (DLB). <b><i>Methods:</i></b> This is a secondary analysis of the Dementia Study of Western Norway (Demvest). For this study, we analysed a sample of 186 patients, 116 with AD and 70 with DLB. Subjects were included at a time in which mild dementia was diagnosed according to consensus criteria after comprehensive standardized assessment. Frailty was evaluated retrospectively using a frailty index generated from existing data. The cut-off value used to classify an older adult as frail was 0.25. <b><i>Results:</i></b> The prevalence of frailty was 25.81% (<i>n</i> = 48). In the DLB group, 37.14% (<i>n</i> = 26) were classified as frail, compared to 18.97% (<i>n</i> = 22) of those with AD (<i>p</i> < 0.001). The adjusted multivariate analysis revealed an OR of 2.45 (1.15–5.23) for being frail in those with DLB when using AD as the reference group. <b><i>Conclusion:</i></b> Frailty was higher than expected in both types of dementia. The prevalence of frailty was higher in those with DLB compared to AD. This new finding underscores the need for a multi-systems approach in both dementias, with a particular focus on DLB.