Background. Mixed infant feeding practice remains a major setback for effective prevention of mother to child transmission of HIV program and updated evidences on this issue is essential for better interventions. Therefore, this study was aimed at assessing the proportion and associated factors of mixed infant feeding practice among HIV-positive women under care in public health institutions in Gondar city within two years postpartum, Ethiopia, 2017. Methods. A cross-sectional study was conducted on 485 HIV-positive women under care in Gondar City’s health facilities from May 1 to June 30/2017. Data were collected via interviewer administered questionnaire supplemented with chart review, entered into Epinfo version 7.0 and then exported to SPSS version 20.0. Both bivariable and multivariable analyses were done, and the statistical significance of each variable was claimed based on the adjusted odds ratio (AOR) with 95% confidence interval (CI) and its P value ≤0.05. Result. The proportion of HIV-positive women practicing mixed infant feeding was 21.6%. Whereas, about 73.8% and 4.5% of the mothers demonstrated exclusive breastfeeding and exclusive replacement feeding, respectively. Mixed infant feeding practice was independently predicted by lack of antenatal care (AOR=6.9; 95% CI: 3.4, 14.1) and home delivery (AOR=2.8; 95% CI: 1.4, 5.4). Conclusion. The magnitude of mixed infant feeding practice was higher than the reports of many other studies, and its predictors were connected to poor adherence to maternal health care service utilization. Hence, stakeholders need to work more on ANC and facility delivery service coverage.