Background: Mental illness, multi-morbidity, and socio-economic inequalities are some of the main challenges for the public health system nowadays, and are further aggravated by the process of population aging. Therefore, it is widely accepted that health systems need to focus their strategies for confronting such concerns. With guaranteed access to health care services under universal coverage in many health systems, it is expected that all services be provided equally to patients with the same level of need. Methods: In this paper, we explore the existence of inequalities in the access to services of patients with mental illness taking into account whether they are multimorbid patients, their socioeconomic status, and their age. We take advantage of a one-year (2010–2011) database on individual healthcare utilization and expenditures for the total population (N = 2,262,698) of the Basque Country. Results: More comorbidity leads to greater inequality in prevalence, being the poor sicker, although with age, this inequality decreases. All health services are more oriented towards greater utilization of the poor and sicker, particularly in the case of visits to specialists and emergency care. Conclusions: Mental health inequalities in prevalence have been identified as being disproportionally concentrated in the least affluent areas of the Basque Country. However, inequalities in the utilization of publicly-provided health services present a pro-poor orientation. As this region has adopted a system-wide transformation towards integrated care, its mental health delivery model offers excellent potential for international comparisons and benchlearning.