Background Preeclampsia and gestational diabetes mellitus (GDM) are common pregnancy complications associated with significant maternal and neonatal morbidity. Objectives This study aims to evaluate and compare maternal and neonatal outcomes in pregnancies complicated by preeclampsia, GDM, and the coexistence of both conditions. Methods This retrospective observational study included 230 pregnant women who delivered at Shalamar Hospital, Lahore, Pakistan, between January 2023 and December 2024. The participants were divided into three groups: preeclampsia only (n = 80), GDM only (n = 75), and both conditions (n = 75). Results The coexistence of preeclampsia and GDM was associated with the highest rates of cesarean section (82.7%, n = 62), preterm birth (46.7%, n = 35), neonatal intensive care unit (NICU) admission (49.3%, n = 37), and neonatal hypoglycemia (18.7%, n = 14). Maternal complications, including intensive care unit (ICU) admission (10.7%, n = 8) and postpartum hemorrhage (9.3%, n = 7), were also more frequent in this group. Logistic regression confirmed that combined preeclampsia and GDM independently predicted adverse outcomes such as cesarean delivery (odds ratio {OR}: 2.4), NICU admission (OR: 2.1), and preterm birth (OR: 1.9). Conclusion It is concluded that the co-occurrence of preeclampsia and GDM significantly worsens both maternal and neonatal outcomes compared to either condition alone. Early diagnosis, multidisciplinary management, and targeted interventions are crucial to mitigating these risks and improving perinatal care.