Objective: An extensive body of research supports the relationship between adverse mental health and chronic illness diagnosis. Similarly, previous studies indicate an association between symptoms of Posttraumatic Stress Disorder (PTSD) and a variety of chronic illnesses. The objective of the current study was to investigate how social factors such as social support, loneliness, and social isolation are related to a variety of mental health indicators among adults with at least one chronic illness. Method: The current study utilised two samples: sample one was drawn from the English Longitudinal Study of Ageing (ELSA) (N = 3,771) and sample two was a primary sample of individuals diagnosed with multiple sclerosis (MS) (N = 28). In study one, standard multiple regression analyses investigated how gender, age, relationship status, self-reported health, social engagement, and loneliness were related to levels of depression, psychological distress, and satisfaction with life. Similarly, standard multiple regression analysis examined how gender, age, number of traumatic experiences, social connectedness, and loneliness were associated with levels of complex posttraumatic stress disorder (CPTSD) in study two. Results: The results of the multiple regression analyses in study one found that loneliness was most strongly associated with depression, psychological distress, and satisfaction with life. Correspondingly, study two also showed that loneliness was significantly and positively associated with CPTSD. Conclusion: The present study highlights the adverse impact of loneliness on mental health among individuals diagnosed with a chronic illness. These findings support the need for further investigation into how feelings of loneliness, in particular chronic illness groups, may influence symptoms of CPTSD. The clinical implications of this study highlight the necessity for clinicians to identify and address feelings of loneliness among individuals with chronic illnesses. Additionally, the findings of the current study support the requirement for interventions to be in place for chronically ill patients targeting feelings of loneliness and psychological distress.