Lumbar degenerative disease and dementia are increasing in super-aging societies and are both related to physical dysfunction and pain. However, the relationship between these diseases remains unclear. This cross-sectional study aimed to investigate the comorbidity rates of lumbar spinal canal stenosis (LSS) and mild cognitive impairment (MCI) and clarify the association between LSS presence, lumbar symptoms, and quality of life (QOL) related to low back pain and cognitive impairment in the Japanese population. We enrolled 336 participants (men 124; women 212; mean age 72.2 years) from a medical checkup program. LSS was diagnosed using a self-administered questionnaire, and lumbar symptoms were evaluated using the visual analog scale (low back pain, and pain and numbness of the lower limb). QOL related to low back pain was evaluated using the Japanese Orthopedic Association Back-Pain Evaluation Questionnaire (JOABPEQ: pain, and lumbar, and gait function). Radiological lumbar degeneration was classified using Kellgren-Lawrence grading and lateral radiographs of the lumbar spine. Cognitive function was measured using the Mini Mental State Examination (MMSE), and MCI was defined by a summary score of MMSE ≤27. Logistic and multiple linear regression analyses were performed to analyze the association between MCI, summary score of MMSE, and lumbar degenerative disease. The comorbidity rate of MCI and LSS was 2.1%, and the rate of MCI was 41% in participants with LSS. Lumbar function in JOABPEQ was associated with MCI. The presence of LSS and lumbar function in JOABPEQ were associated with MMSE. Over one-third of the people with LSS had MCI. The presence of LSS and deterioration of QOL due to low back pain were related to cognitive impairment. We recommend evaluating cognitive function for patients with LSS because the rate of MCI was high in LSS participants.