Background The health of caregivers is as important as that of the elderly they care for. Although several studies have stressed the necessity and importance of reducing the care burden on formal caregivers and increasing the quality of their working lives, this issue has not been addressed both qualitatively and quantitatively from the caregivers' perspective. Aim This study is a comprehensive assessment of the effects of formal caregivers' care burden on the quality of their working lives. Method A convergent design of mixed methods was used. This study was conducted with 110 formal caregivers at an Elderly Care and Rehabilitation Center between 1 and 31 January 2019. Additional semi-structured interviews were conducted with 10 of the respondents. Descriptive statistics were used for quantitative analysis and thematic statistics for qualitative analysis. The quantitative and qualitative data were merged and integrated for a mixed-method analysis. Results The relationship between the quality of working life and care burden was statistically significant for the formal caregivers. The results of the interviews supported this relationship. Four themes emerged from the interviews: the effects of caregiving on care staff, the effects of caregiving on caregivers' daily lives, the requirements for providing better care and caregivers' description of caregiving in a few words. Conclusion Given that the care burden of formal caregivers affects the quality of their working lives, this burden should be assessed on a regular basis and efforts needed to be made to reduce it and improve the quality of their working lives.