Background: Chronic non-communicable diseases (NCDs) represent a growing public health burden in urban India, with lifestyle behaviours increasingly recognised as modifiable risk factors. The Knowledge-Attitude-Practices (KAP) framework, integrated with the Health Belief Model (HBM), provides a structured theoretical lens for understanding multi-dimensional behavioural determinants of health outcomes. Objective: To examine dietary habits, nutritional patterns, and lifestyle behaviours as predictors of chronic disease symptom severity across four age groups (18–30, 31–45, 46–60, and 61+ years) in Delhi NCR using an integrated KAP-HBM framework. Methods: A cross-sectional survey was conducted with 100 participants from Delhi NCR, stratified across four age groups and four occupational categories. A 68-item self-administered questionnaire assessed six behavioural domains. Non-parametric tests (Spearman's correlation, Kruskal-Wallis H, Mann-Whitney U) and multiple linear regression were employed. Cronbach's alpha confirmed scale reliability. Results: Psychological well-being was the only significant independent predictor of chronic disease symptom severity (β = 0.502, p < 0.001, R² = 0.218). Significant age-group differences emerged for dietary practices (H = 10.97, p = 0.012), physical activity (H = 10.67, p = 0.014), and psychological well-being (H = 8.42, p = 0.038). Older adults reported healthier dietary and activity patterns but poorer psychological well-being. No significant gender differences were observed. Conclusion: Psychological well-being is the most critical independent predictor of chronic disease symptoms in this urban sample, underscoring the need for integrated mental health components within NCD prevention programmes targeting diverse age groups in Delhi NCR.