Research shows that people with cystic fibrosis are more prone to suffer from psychological problems than healthy people; thus, the outbreak of the COVID-19 pandemic in Poland could have had an impact on their mental health. To assess this impact, we examined the mental health of patients before and during the pandemic. Survey participants were asked to fill in questionnaires that consisted of Beck Depression Inventory (BDI), 12-Item General Health Questionnaire (GHQ-12) and Cystic Fibrosis Questionnaire-Revised (CFQ-R; for the purpose of the study, an emotional functioning domain was used) during their hospital visits. A total of 81 patients took part in the study: 39 before the COVID-19 pandemic (BP) and 42 during the COVID-19 pandemic (DP). Patients’ medians were lower for the BDI, GHQ-12 and higher for the emotional domain of CFQ-R during the pandemic (3, 6, 75 vs. 4, 10, 73.33). Fewer patients felt that their mental health had deteriorated during the pandemic (Δχ2 = 7.723; p = 0.005), and GHQ-12 scores were lower in the DP group (Z = −3.044; p = 0.002). No significant differences were found between groups in terms of experiencing depressive symptoms (Δχ2 = 1.036; p = 0.309). It was found that patients with cystic fibrosis from our study group not only maintained but also improved their mental health state during the COVID-19 pandemic.