A major challenge in downstream purification of monoclonal antibodies (mAb) is the removal of host cell proteins (HCPs). Previous studies have shown that cell culture conditions significantly impact the HCP content at harvest. However, it is currently unclear how process conditions affect physiological changes in the host cell population, and how these changes, in turn, cascade down to change the HCP profile. We examined how temperature downshift (TDS) to mild hypothermia affects key upstream performance indicators, that is antibody titre, HCP concentration and HCP species, across the cell culture decline phase and at harvest through the lens of changes in cellular behavior. Mild hypothermic conditions introduced on day 5 of fed-batch Chinese hamster ovary (CHO) cell bioreactors resulted in a lower cell proliferation rate but larger percentages of healthier cells across the cell culture decline phase compared to bioreactors maintained at standard physiological temperature. Moreover, the onset of apoptosis was less evident in mild hypothermic cultures. Consequently, mild hypothermic cultures took an extra 5 days to reach an integral viable cell concentration (IVCC) and antibody yield similar to that of the control at standard physiological temperature. When cell viability dropped below 80%, mild hypothermic cell cultures had a reduced variety of HCP species by 36%, including approximately 44% and 27% lower proteases and chaperones, respectively, despite having similar HCP concentration. This study suggests that TDS may be a good strategy to provide cleaner downstream feedstocks by reducing the variety of HCPs and to maintain product integrity by reducing the number of proteases and chaperones.