Background Appropriate complementary feeding has the potential to prevent 6 percent of all under five deaths particularly in developing countries. However, infant and young child feeding practices in Ethiopia are suboptimal. Data on complementary feeding practices in Horro district are also lacking. Therefore, this study aimed to determine magnitude and determinants of appropriate complementary feeding practice among mothers of children age 6–23 months in Horro district, Western Ethiopia. Methods Community based cross sectional study was conducted from February to March 2016 in six kebeles of Horro district, Western Ethiopia. A total of 325 mothers of children age 6–23 months were included in the study. Data were collected using pre-tested interviewer administered structured questionnaire; entered into EPI-INFO version 3.5.4 and analyzed using SPSS version 20. Odds ratio was calculated with 95% CI to identify determinants of appropriate complementary feeding practice. P-values less than 0.05 were considered as statistically significant. Results The magnitude of appropriate complementary feeding practice in the study area was 9.91%. Lower age of child (6-11months) [AOR: 0.195, 95% CI: (0.045–0.846)], having no formal education [AOR = 0.115:95% CI: (0.002, 0.290)] and giving birth for the first time [AOR = 0.271:95% CI: (0.011, 0.463)] were factors negatively associated with appropriate complementary feeding practice. Conclusion Only one tenth of mothers practiced appropriate complementary feeding. This strongly calls for sustained nutrition education targeting uneducated mothers, mothers who gave birth for the first time and those with very young children to improve the practice.