Background Surgical systems change throughout the year, and were forced to adapt to COVID-19. This study aimed to investigate the impact of month of the calendar year and COVID-19 pandemic on general surgery outcomes. Methods A retrospective cohort study was conducted including patients admitted under general surgery services of two hospitals in South Australia over six years. To evaluate outcomes of length of hospital stay, in-hospital mortality, and complications, regression models adjusted for confounders were developed for calendar month of hospital discharge and period before or after COVID-19. Results 43,633 general surgery admissions were included, 20,238 (46.4%) before COVID-19 (<2020), and 23,390 (53.6%) after. There was a statistically significant association between calendar month of hospital discharge, and period before versus after COVID-19, and: length of hospital stay (p=0.0002 and p=0.0005 respectively), the presence of complications (both p<0.0001), and complication grade (both p<0.0001). However, there was no significant association between calendar month of hospital discharge, and period before versus after COVID-19, and in-hospital mortality (p=0.6182 and p=0.3572, respectively). Multiple differences in study outcomes were found in comparisons between temporally-adjacent discharge months of the year. Conclusions Discharge during certain months of the year, and period before or after COVID-19, may be associated with differences in length of hospital stay and complications, but not in-hospital mortality, across general surgery. To inform infrastructure capacity building, future research should explore a potential causative relationship between the identified factors, as this was outside the scope of the present study.