Background: This study investigated how cumulative lifetime stress, as measured by the Stress and Adversity Inventory (STRAIN) scale, relates to salivary cortisol levels in temporomandibular disorders (TMD) patients compared to controls. Furthermore, to determine which specific lifetime stress domains are the strongest predictors of TMD. Methods: The study was conducted with 110 participants (55 TMDs patients, 55 controls). Lifetime stress was assessed using the STRAIN questionnaire, and salivary cortisol levels were measured at two time points (7 AM and 10 AM) using Enzyme-Linked Immunosorbent Assay (ELISA). Statistical analyses included t-tests, Analysis of variance (ANOVA) and multiple regression to identify significant stress predictors for TMD. Results: The TMDs patients had significantly higher stress scores (11.10 ± 3.26) compared to the controls (1.43 ± 0.99) (p = 0.001). Myalgia showed highest stress levels (11.69 ± 3.72), while patients with myofascial pain had the lowest (8.80± 1.14) (p = 0.043). Cortisol levels were highest in the of disc displacement without reduction with limited mouth opening (DDWoR with LO) group (82.49 ± 124.34) and lowest in myalgia patients (4.69 ± 3.90) (p = 0.001). Significant stress predictors for TMDs included relationship stress (p = 0.04), humiliation (p = 0.02), marital/partner stress (p < 0.001) and death-related stress (p = 0.01). Conclusions: TMDs patients experience significantly higher lifetime stress and cortisol levels than controls. Myalgia patients showed a complex psychological and physiological stress link, whereas the DDWoR with LO subgroup exhibited a distinct physiological stress response. Specific life stressors, particularly relationship- and partner-related stress, are key predictors of TMDs. These findings reinforce the importance of a biopsychosocial approach in understanding and managing TMDs. Future research should focus on longitudinal and interventional studies to further elucidate causal mechanisms and effective therapeutic strategies.