Background: Professional burnout in the medical community has been present for a long time, also among mental health professionals. The aim of the study was to examine the links between loneliness, complaining and professional burnout among medical personnel in psychiatric care during a pandemic. Loneliness and complaining of the medical staff are not documented in the literature well enough. Methods: Oldenburg Burnout Questionnaire, the Loneliness Scale, the Complaint Questionnaire and author’s questionnaire. The respondents: 265 medical employees—doctors (19.2%), nurses (69.8%), paramedics (4.9%), medical caregivers (5.7%). Results: Loneliness and complaining are significant predictors of exhaustion. The model explains 18% of exhaustion variance. Loneliness, complaining and job seniority are also predictors of disengagement; the model allows to predict 10% of the variance of disengagement. Women are more prone to complain. Complaining significantly correlates with direct support from management. A high rate of loneliness correlates, in a statistically significant way, with worse work organization, less management support, worse atmosphere in the team and with more irresponsible attitudes of colleagues. Conclusions: Loneliness and complaining can be used to predict occupational burnout. Women and people without management support complain more often. Loneliness is connected with bad work organization and bad cooperation in a team.