Background: Malaria is an infectious disease which has been globally targeted for elimination in at least 35 of 90 endemic countries by 2030. Most successful malaria elimination country programs have engaged the private health sector in an effort to identify, document, investigate, provide effective treatment, and follow-up all cases. However, there has been limited rigorous research showing evidence of adherence among healthcare providers of the formal private health sector with national malaria diagnosis and treatment guidelines in Ethiopia, starting from malaria control to elimination phases. The aims of this study were to investigate and explain the level of adherence to malaria diagnosis and treatment guidelines among healthcare providers working in formal private health facilities in north-western Ethiopia.Methods : An explanatory sequential mixed method design was conducted in the West Gojjam Zone of Ethiopia. Quantitative data were extracted from 1,650 medical records of adult uncomplicated malaria outpatients served in 11 private-for-profit health facilities. In addition, using a qualitative approach, 33 in-depth interviews (IdIs) with healthcare providers were conducted. All interviews were audio-recorded, transcribed verbatim, and analyzed using eight steps.Results: Of 1,650 suspected malaria cases in adult outpatients, 80.6% (1,330/1,650) were screen tested using microscopy and the remainder19.4% (320/1,650) were tested using multi-species rapid diagnosis tests (RDTs). Hence, the results revealed that private healthcare providers universally adhered to diagnosis guidelines. In addition, after following-up and excluding other causes of fever, 4.1% (56/1,376) patients were clinically diagnosed with uncomplicated malaria. Despite this, the proportion of private healthcare provider adherence with confirmed malaria case treatment guidelines was 20.9% (69/330). In addition, 1,320 (95.9%) of adult outpatients with negative laboratory results were not treated. Some of the identified determinant factors for suboptimal adherence of healthcare providers to malaria guidelines were interruptions in supply and lack of availability of recommended antimalarial drugs, lack of availability of quality assured laboratory supplies, and poor knowledge of the recommendations of the national standards.Conclusions: Private healthcare providers adhered to universal parasitological diagnosis, providing comprehensive counseling, and linking patients with community health workers. In addition, almost all laboratory negative patients were not treated with antimalarial drugs. However, only one-fifth of confirmed patients were treated in line with national guideline recommendations. Therefore, building the capacity of private healthcare providers and ensuring the availability of all nationally recommended drugs and supplies in private health sector facilities is recommended to improve the quality of services.