Improved understanding of multimorbidity (MM) treatment adherence in primary health care (PHC) in Brazil is needed to achieve better healthcare and service outcomes. This study explored experiences of healthcare providers (HCP) and primary care patients (PCP) with mental-physical MM treatment adherence. Adults PCP with mental-physical MM and their primary care and community mental health care providers were recruited through maximum variation sampling from nine cities in São Paulo State, Southeast of Brazil. Experiences across quality domains of the Primary Care Assessment Tool-Brazil were explored through semi-structured in-depth interviews with 19 PCP and 62 HCP, conducted between April 2016 and April 2017. Through thematic conent analysis ten meta-themes concerning treatment adherence were developed: 1) variability and accessibility of treatment options available through PHC; 2) importance of coming to terms with a disease for treatment initation; 3) importance of person-centred communication for treatment initiation and maintenance; 4) information sources about received medication; 5) monitoring medication adherence; 6) taking medication unsafely; 7) perceived reasons for medication non-adherence; 8) most challenging health behavior change goals; 9) main motives for initiation or maintenance of treatment; 10) methods deployed to improve treatment adherence. Our analysis has advanced the understanding of complexity inherent to treatment adherence in mental-physical MM and revealed opportunities for improvement and specific solutions to effect adherence in Brazil. Our findings can inform research efforts to transform MM care through optimization.