Background For patients admitted to the intensive care unit (ICU) with sepsis, mobilization therapy during ICU stay can improve their outcomes during and after the ICU stay. However, little is known about the optimal timing of introducing mobilization therapy. Methods This is a retrospective cohort study using data from a tertiary medical center in Japan during 2013–2017. We included patients aged ≥ 18 years who were admitted to the ICU with sepsis based on the Sepsis-3 criteria. We defined early mobilization (EM) as the rehabilitation at the level of sitting on the edge of the bed or more within the first 3 days of the patients’ ICU stay. Patients were divided into the EM and non-EM groups. The primary outcomes were in-hospital mortality and ambulatory dependence at hospital discharge. We estimated the effects of EM by stabilized inverse probability weighting (sIPW). We then tested alternative definitions of EM by changing the cutoff in days to mobilization by 1-day increments from 2 to 7 days to investigate the optimal timing of mobilization. Results Our study sample consisted of a total of 296 septic patients, including 96 patients in the EM group and 200 patients in the non-EM group. In the sIPW model, the adjusted OR for in-hospital mortality in the EM group compared to the non-EM group was 0.22 [95% CI 0.06–0.88], and the adjusted OR for ambulatory dependence at the hospital discharge was 0.24 [95% CI 0.09–0.61]. When alternative definitions of EM were tested, patients who achieved mobilization within the first 2–4 days of their ICU stays had better outcomes. Conclusions Achieving mobilization within the first 3 days of ICU stay was significantly associated with better outcomes. Patients with sepsis might benefit most from achieving mobilization within 2–4 days. Further studies are warranted to validate the findings. Supplementary Information The online version contains supplementary material available at 10.1186/s40560-022-00613-8.