Background The incidence of eclampsia and its adverse maternal outcomes are very high in developing countries, particularly in Subsaharan African Countries. Identifying predictors for adverse maternal outcomes of eclampsia has paramount importance for helping health care providers to optimize their management outcomes. Therefore, this study aimed to assess the incidence of adverse maternal outcomes of eclampsia and its determinant factors. Methods A retrospective follow-up study design was applied. The data were extracted from patient charts using a structured, pre-tested, questionnaire. Descriptive analyses (frequencies, means, and standard deviation) were calculated, and bi-variable and multivariable logistic regression models were used to testing the association between independent variables and an outcome variable. After the data were coded and entered into Epi-Info Version 7.2 Software, the data were analyzed using STATA Version 14 Statistical Software. Results The magnitude of eclampsia was 5.36 per 1000 pregnancies (95% CI: 4.72, 6.10). The incidence of adverse maternal outcomes in eclamptic mothers was 53.7% (95% CI: 47.02, 60.24%). After adjusting for covariates maternal age 30–34, AOR 5.4 [95% CI = 1.02, 28.6]; age above 34, AOR 10.5 [95% CI = 1.3, 88.6]; gravidity 2–4, AOR 0.3 [95% CI = 0.1, 0.9]; 10 or more convulsions, AOR 4.6 [95% CI = 1.4, 14.9]; mild pyrexia, AOR 20.4 [95% CI = 3.7, 112.7]; moderate pyrexia, AOR 14.6 [95% CI = 1.7125.1]; platelet count below 50,000 cells/mm3, AOR 34.9 [95% CI = 3.6, 336.2]; platelet count between 50,000 and 99,000 cells/mm3, AOR 24.5 [95%CI = 5.4111.6]; and stillbirth of the current pregnancy, AOR 23.2 [95%CI = 2.1257.5] were strong predictors of adverse maternal outcomes in eclamptic mothers. Conclusions The incidence of adverse maternal outcomes of eclampsia was found to be high compared to similar studies discussed in this study. This study recommends early identification of patients with the risk factors (having many convulsions, high body temperature, low platelet count, patient age above 30 years, and 2–4 pregnancies), strengthening the referral system, and advocation of research on the area of adverse maternal outcomes and thereby encourage evidence-based medicine.