Disordered eating behaviors are common in people with a serious mental illness (SMI) such as schizophrenia, bipolar disorder and major depressive disorder. This study employed qualitative exploration to understand the perceived determinants of eating behaviors, in particular those connected to disordered eating patterns, in people with SMI. In total, 28 semi-structured interviews were conducted in a consecutive sample of people with SMI under treatment in local mental health services in Australia (n = 12), Germany (n = 8) and Austria (n = 8) (mean age: 43.3 years, proportion of female participants: 61%, proportion of participants with ICD-10 F2 diagnosis: 57%, proportion of participants with ICD-10 F3 diagnosis: 64%). A thematic analysis approach, the framework method, was applied using MAXQDA 2020. Three main themes of determinants were derived: (i) impacts to daily functioning, (ii) disrupted physical hunger cues and (iii) emotional hunger. For impacts to daily functioning, the following themes emerged: lack of daily structure, time and drive, and difficulty planning ahead. For physical hunger, themes emerged for disrupted hunger and satiety cues, and mindless eating. All motives listed in the Palatable Eating Motives Scale (PEMS), i.e., coping, reward, social and conformity, have been reported by participants to be drivers for their emotional eating behavior. Subsequent reported behaviors were eating too much or too little, binge eating, night eating and food cravings. We conclude that interprofessional approaches should target daily functioning, disrupted physical hunger cues and emotional eating to reduce disordered eating behaviors in people with SMI.