Introduction: Mental disorders are one of the most common and disabling health conditions worldwide. There is however no consensus on the best practice of system level mental health services (MHS) provision, in order to prevent e.g. mood disorder disability pensions (DPs). We analyzed the MHS provision between Finland’s three largest hospital districts Helsinki and Uusimaa (HUS), Southwest Finland and Pirkanmaa, with known differences in mood disorder DP risk but presumably equal rates of mood disorder prevalence. Methods: We used public MHS data analyzed with the standardized DEscription and Evaluation of Services and DirectoriEs for Long Term Care (DESDE-LTC) mapping tool, focusing on all MHS, outpatient care provision, local services without and with gatekeeping, and centralized services. We also collected demographic data based on the European Socio-Demographic Schedule (ESDS). As a novel approach, the Gini-Simpson Diversity Index (GSDI) was calculated for the districts. Results: Evident differences were observed regarding the districts’ MHS factors. As the hospital district with lower DP risk, HUS was characterized by the highest level of regional socioeconomic prosperity as well as high service richness and diversity. With a nationally average DP risk, Southwest Finland had the highest number of MHS personnel in full-time equivalents (FTE) per 100 000 inhabitants. Pirkanmaa, with a higher DP risk, had overall the lowest service richness and the lowest FTE of the three districts in all MHS, outpatient care and local services with gatekeeping. Conclusions: Our findings indicate that greater richness and diversity of MHS, especially in outpatient and community-based settings, may serve as indicators of a balanced, high-quality service system that is more effective in preventing mood disorder DP and meeting the different needs of the population. In addition, the need for sufficient resourcing in all MHS and outpatient services is indicated. We suggest using diversity indices to complement the measuring and reporting of regional service variation.