Background: Critically ill patients in intensive care units (ICU) are frequently administered broad-spectrum antibiotics (e.g., carbapenems or piperacillin/tazobactam) for suspected or confirmed infections. This retrospective cohort study aimed to describe the use of carbapenems and piperacillin/tazobactam in two international, prospectively collected datasets. Methods: We conducted a post hoc analysis of data from the “Adjunctive Glucocorticoid Therapy in Patients with Septic Shock” (ADRENAL) trial (n = 3713) and the “Antimicrobial de-escalation in the critically ill patient and assessment of clinical cure” (DIANA) study (n = 1488). The primary outcome was the proportion of patients receiving initial antibiotic treatment with carbapenems and piperacillin/tazobactam. Secondary outcomes included mortality, days alive and out of ICU and ICU length of stay at 28 days. Results: In the ADRENAL trial, carbapenems were used in 648 out of 3713 (17%), whereas piperacillin/tazobactam was used in 1804 out of 3713 (49%) participants. In the DIANA study, carbapenems were used in 380 out of 1480 (26%), while piperacillin/tazobactam was used in 433 out of 1488 (29%) participants. Mortality at 28 days was 23% for patients receiving carbapenems and 24% for those receiving piperacillin/tazobactam in ADRENAL and 23% and 19%, respectively, in DIANA. We noted variations in secondary outcomes; in DIANA, patients receiving carbapenems had a median of 13 days alive and out of ICU compared with 18 days among those receiving piperacillin/tazobactam. In ADRENAL, the median hospital length of stay was 27 days for patients receiving carbapenems and 21 days for those receiving piperacillin/tazobactam. Conclusions: In this post hoc analysis of ICU patients with infections, we found widespread initial use of carbapenems and piperacillin/tazobactam in international ICUs, with the latter being more frequently used. Randomized clinical trials are needed to assess if the observed variations in outcomes may be drug-related effects or due to confounders.