Background: Diabetes mellitus is a significant global public health issue, often exacerbated by microvascular and macrovascular damage, resulting in heightened morbidity and mortality. Recognizing modifiable and non-modifiable characteristics linked to diabetic complications is crucial for enhancing preventative efforts and directing personalized medication. Objectives: This study aimed to identify demographic, clinical, lifestyle, and treatment-related factors associated with microvascular and macrovascular complications among adults with diabetes mellitus. Methods: A cross-sectional analytical study was performed at the National Diabetes Center, Al-Mustansiriya University, from September to December 2025, involving 100 persons with diabetes mellitus. Data on sociodemographic characteristics, lifestyle factors, smoking status, clinical parameters, glycaemic control (HbA1c), treatment modalities, and diabetic complications were collected. Microvascular and macrovascular complications were defined as composite binary outcomes. Bivariate analyses were performed using appropriate parametric and non-parametric tests. Multivariable logistic regression models were constructed to identify independent predictors of microvascular and macrovascular complications, adjusting for potential confounders. Results: The prevalence of microvascular and macrovascular complications increased with advancing age and longer diabetes duration. Poor glycaemic control was associated with a higher burden of complications. In multivariable analysis, age was independently associated with microvascular complications, while diabetes duration showed a significant association with macrovascular complications. Smoking status and insulin-based therapy demonstrated trends toward higher complication risk after adjustment for demographic and clinical variables. Conclusions: Age, duration of diabetes, and glycaemic control are key determinants of diabetic complications. Biomarker-based clinical parameters combined with lifestyle and treatment factors provide valuable insight into disease staging and risk stratification. These findings support the importance of early intervention and individualized pharmacotherapeutic strategies to reduce long-term complications in patients with diabetes mellitus.