A 65-year-old man visited a primary care hospital with a continued fever of 38°C for 3 days. As his fever did not improve until 8 days after, he was admitted into another acute care hospital, where his respiratory condition rapidly worsened. Therefore, the patient was transferred to our hospital. On the day of transfer (day 1) he was started on mechanical ventilation. COVID-19 was diagnosed using a polymerase chain reaction assay 6 days after admission (day 6). The rehabilitation therapy was begun on day 6. The initial rehabilitation programs focused on positioning and postural drainage. The patient was extubated on day 19 and he began standing and stepping on the same day. Gait exercises began on day 22, and endurance training was initiated on day 28. The patient was discharged from our hospital on day 34 as he met the physical function milestones. One month after discharge, the Medical Research Council sum score and Barthel Index had each improved; therefore, muscle strength and daily activities had returned to normal. It was assumed that mobilization should be performed as soon as possible after the end of sedation during the acute phase of severe COVID-19 infection in patients receiving mechanical ventilation.