Background and Objective: The goal was to measure Health Related Quality of Life (HRQoL) in individuals with different types of dementia, mild cognitive impairment (MCI) and healthy controls (HC), and assess agreement levels between participants and proxies. Materials and Methods: A sample of 136 participants were recruited; thirty-seven with Alzheimer’s Disease Dementia (AD), 19 with Dementia with Lewy Bodies (DLB), 12 with Frontotemporal Dementia (FTD), 37 with MCI and 31 HC. HRQoL was measured via the 12-Item Short Form Health Survey, version 2 (SF-12-v2), separately for participants and proxies. Two groups (Individuals with cognitive decline versus healthy controls) were matched for sociodemographic variables. Differences for discrepancy rates between both groups were measured using t-test. Participant and proxy agreements were measured via Intraclass Correlation Coefficients (ICC). Linear regression analyses were performed to examine what variables explained better the variance observed. Results: Patients with DLB and FTD showed the lowest levels of HRQoL, while AD and HC showed the highest. No statistically significant differences were found between discrepancy scores from cognitive decline groups and healthy controls. ICC indicated high agreement between patients and proxies for the groups with cognitive decline, while agreements achieved in HC were lower and only in physical indices. GDS score accounted for 8.3% of the variance of proxies’ rating on Mental Summary Composite Score (MSC). Conclusion: HRQoL physical and mental summaries are more reliable in groups with cognitive decline. Healthy controls and their proxies show lower agreement, with proxies reporting lower levels than normal for some relevant indices.