Background: The World Health Organization recommends testing of all HIV exposed infants at 4–6 weeks of age to optimize detection of intrauterine, intrapartum, and early postnatal HIV transmissions. However, the global coverage of early infant diagnosis remains low. With less than 25% national coverage, the progress of this early diagnosis in Ethiopia is one of the slowest in the World. Furthermore, local studies are limited to determine the status and identify the associated factors in many parts of the country in general and in West Shoa Zone in particular. Thus, this study aimed to fill this gap.Methods: A mixed method study by using retrospective record review of four years facility data of 342 mother-infant pairs supplemented by qualitative method was conducted in West Shoa Zone, Oromia Region, Ethiopia in 2018. The quantitative data related to maternal and HIV exposed infants’ cares were collected by using questionnaire. The qualitative data related to facility related barriers were collected by key-informants interview with selected mothers and service providers. Factors associated with early infant diagnosis of HIV infection were identified by using multivariable logistic regression analysis. The qualitative data were analyzed thematically and triangulated with the quantitative findings. Results: The status of early infant diagnosis was 58.5% (95%CI: 53.3%, 63.7%). Attending secondary education or above (AOR=2.41; 95%CI: 1.54, 3.28), having <4 living children (AOR=4.76, 95%CI: 02.56, 9.09), knowing HIV sero-status during or before pregnancy (AOR=6.24, 95%CI: 2.40, 10.08) and sero-status disclosure (AOR=8.30, 95%CI: 3.30, 20.60) were significantly associated with early infant diagnosis. Attending ANC visit (AOR=5.32; 95%CI: 2.53, 8.11), giving birth in health facility (AOR=62; 95%CI: 3.39, 11.85) and Neverapine provision at enrolment (AOR=6.05; 95%CI: 2.48, 14.73) were also significantly associated with early infant diagnosis of HIV infection.Conclusion: The status of early infant diagnosis in the study area is low to achieve the national target of 95% coverage by 2020. Maternal socio-demography, maternal behavior in knowing sero-status and disclosing and using services during pregnancy and delivery were among the determinants of early infant diagnosis. Promoting ANC and PMTCT cascade through behavioral interventions and fulfilling the essential equipments and supplies are required.