Background: Apart from clinical experience and theoretical considerations, there is a lack of evidence that the level of adherence to in-hospital mobilization protocols is related to functional recovery in patients after resection for lung cancer. The objectives of the study were to determine (1)the relationship between adherence to the in-hospital mobilization protocol and physical fitness at hospital discharge and (2)the value of physical fitness measures at discharge in predicting physical functioning 6 weeks and 3 months postoperatively. Methods: This observational study included 62 patients who underwent surgical resection for lung cancer. Adherence to the in-hospital mobilization protocol was abstracted from patients’ records. Physical fitness measures before the operation and at hospital discharge included handgrip strength, 30-second sit-to-stand test, and 6-minute walk test (6MWT). Self-reported physical functioning was assessed preoperatively and 6 weeks and 3 months postoperatively, using the Medical Outcome Study 36-Item Short Form (SF-36)Physical Function subscale (RAND Corp, Santa Monica, CA). Linear regression analyses were used to estimate the relationships of interest, adjusting for potential confounders. Results: Level of adherence to the mobilization protocol was significantly and independently related to handgrip strength, sit-to-stand test, and 6MWT at discharge. Handgrip strength and 6MWT at discharge significantly predicted SF-36 Physical Function at 6 weeks and 3 months postoperatively. The sit-to-stand test only predicted SF-36 Physical Function at 6 weeks. Conclusions: Suboptimal postoperative mobilization after surgical resection for lung cancer negatively affects physical fitness at discharge. Our results underline the importance of adherence to early postoperative mobilization protocols. Measuring physical fitness at discharge may be useful to inform clinicians on elective referral of patients for postdischarge rehabilitation.