Lumbar canal stenosis (LCS) has been reported as a precipitating factor by which a tethered spinal cord, which is asymptomatic during childhood, develops into tethered cord syndrome (TCS) in adulthood. However, only a few reports on surgical strategies for such cases are available. A 64-year-old woman presented with unbearable pain in the left buttock and dorsal aspect of the thigh approximately 1 year ago. Magnetic resonance imaging showed cord tethering with a filar-type spinal lipoma and LCS due to the thickening of the ligamentum flavum at the L4-5 vertebral level. Five months after the decompressive laminectomy for the treatment of LCS, an untethering surgery was performed at the dural cul-de-sac at the S4 level. The severed end of the filum was elevated rostrally by 7 mm, and the pain subsided postoperatively. This case study shows that surgeries for both lesions should be indicated for adult-onset TCS triggered by LCS.