Background and Objectives: The coronavirus disease 2019 (COVID-19) preventive measures affected various aspects of people’s lives, while also representing an important risk factor for people’s mental health. In the present study, we examined the negative psychological consequences of the preventive measures on people’s mental health and the protective factors that strengthened their mental health and well-being during the pandemic. Materials and Methods: A study, using a combination of qualitative and quantitative methods based on a Delphi protocol, was conducted with a sample of Slovenian professionals who worked with people from different demographic groups (i.e., children and adolescents, emerging adults, the adult working population, the elderly) during the pandemic. We conducted (i) a qualitative study involving semi-structured interviews with 11 professionals and (ii) a quantitative study where 73 professionals completed a structured online questionnaire. Results: Experts recognized the disruption of informal face-to-face social contacts as the measure with the greatest impact on people’s lives across all groups studied, the effect being particularly evident in relation to individuals’ development period and socio-demographic characteristics. An individual’s ability to adapt to change and emotional support provided by family or other close persons contributed significantly to maintaining mental health and well-being during the pandemic. Conclusions: Considering the interplay of various COVID-19-related risk and protective factors for mental health, enabling and promoting the maintenance and development of social relationships (including through alternative pathways) should be a priority aspect of (mental health) intervention for all demographic groups.