ObjectiveEmotion regulation and social identity theorizing provide two influential perspectives on loneliness. From an emotion regulation perspective, loneliness is understood as a negative emotional state that can be managed using emotion regulation strategies. A social identity perspective views loneliness as resulting from a loss or lack of important social groups and related identities. This study aimed to explore the relationships between key constructs drawn from both perspectives, with a view to understanding loneliness in adults with and without a history of mental illness.Design and MethodsParticipants (N = 875) with a mental illness history (MH Hx, n = 217; M_age = 45 years, 59% female) and without a mental illness history (No MH Hx, n = 658; M_age = 47 years, 48% female) completed a survey comprising measures of group membership and connectedness, emotion regulation strategies, and loneliness.ResultsThe MH Hx group reported higher internal affect worsening strategy use and loneliness than those No MH Hx. Hierarchical regressions indicated that the unique contributions of emotion regulation strategies and social identity factors to loneliness were equivalent between the groups. Together, social identity and emotion regulation explained 37% of the variance in loneliness in the No MH Hx subsample and 35% in the MH Hx subsample.ConclusionThese findings suggest that both emotion regulation and social identity had significant unique contributions to the reported loneliness of people when controlling for demographics and each other in those with and without a history of mental illness. Integration of the two frameworks may provide novel avenues for the prevention and management of loneliness.Practitioner points Individuals with a history of mental illness report more use of internal emotion worsening regulation strategies and greater loneliness than those with no such history, but there were no differences in social identity factors. Internal emotion worsening strategies and social support received from others explained the variance in reported loneliness for both those with and without a history of mental illness. Internal emotion improving strategies were significant for those with a history of mental illness, while social support given was significant for those without a history of mental illness. Screening clients for emotion regulation difficulties, social disconnectedness, and loneliness may provide clinicians with an indication of risk for developing psychological distress/disorders.