Objective: The objective of this study was to examine clinical characteristics, cognitive decline, and predictors for time to dementia in prodromal dementia with Lewy bodies with mild cognitive impairment (MCI-LB) compared with prodromal Alzheimer's disease (MCI-AD). Methods: We included 73 MCI-LB patients (12% female; 68 ± 6 years; Mini Mental State Examination, 27 ± 2) and 124 MCI-AD patients (48% female; 68 ± 7 years; Mini Mental State Examination, 27 ± 2) from the Amsterdam Dementia Cohort. Follow-up was available for 61 MCI-LB patients and all MCI-AD patients (3 ± 2 years). We evaluated dementia with Lewy bodies core features, neuropsychiatric symptoms, caregiver burden (Zarit caregiver burden interview), MRI, apolipoprotein genotype, and cerebrospinal fluid biomarkers (tau/Aβ1–42 ratio). Longitudinal outcome measures included cognitive slopes (memory, attention, executive functions, and language and visuospatial functions) and time to dementia. Results: Parkinsonism was the most frequently present core feature in MCI-LB (69%). MCI-LB patients more often had neuropsychiatric symptoms and scored higher on ZARIT when compared with the MCI-AD patients. Linear mixed models showed that at baseline, MCI-LB patients performed worse on nonmemory cognitive domains, whereas memory performance was worse in MCI-AD patients. Over time, MCI-LB patients declined faster on attention, whereas MCI-AD patients declined faster on the Mini Mental State Examination and memory. Cox proportional hazards regressions showed that in the MCI-LB patients, lower attention (hazard ratio [HR] = 1.6; 95% confidence interval [CI], 1.1–2.3) and more posterior cortical atrophy (HR = 3.0; 95% CI, 1.5–5.8) predicted shorter time to dementia. In the MCI-AD patients, worse performance on memory (HR = 1.1; 95% CI, 1.0–1.2) and executive functions (HR = 1.3; 95% CI, 1.0–1.6) were independently associated with time to Alzheimer's dementia. Conclusion: MCI-LB patients have distinct neuropsychiatric and cognitive profiles with prominent decline in attention when compared with MCI-AD patients. Our results highlight the importance of early diagnosis because symptoms already have an impact in the prodromal stages.