OBJECTIVE This study determined whether mobility and functional pain were different among older men and women with chronic low-back pain and varying body mass index levels. METHODS This was a comparative, descriptive study of older adults with obesity with low-back pain (N = 55; 60-85 yrs). The participants were stratified on the basis of body mass index: overweight (25-29.9 kg/m), obese (30-34.9 kg/m), and severely obese (35 kg/m). The participants completed a functional test battery (walking endurance, chair rise, stair climb, 7-day activity monitoring, and gait parameters) and pain ratings with activity ("functional pain"). RESULTS The functional pain scores during walking and stair climb were highest in the severely obese group compared with the overweight group (P < 0.05), but the functional test scores were not found to be significantly different by body mass index. Gait base of support was 36% greater and single/double support times were 3.1%-6.1% greater in the severely obese group compared with the overweight group (P < 0.05). The women had slower chair rise and stair climb times and had slower walking velocity than did the men. Daily step numbers were lowest in the severely obese group compared with the obese and overweight groups (2971 vs. 3511 and 4421 steps per day; P < 0.05) but were not different by sex. Normalized lumbar extensor, abdominal curl, and leg press strength values were lowest in the severely obese group, and the women had 18%-34% lower strength values than did the men for all three exercises (P < 0.05). Lumbar strength was associated with stair climb, chair rise, and walking endurance times. Body mass index was an independent predictor of walking endurance time but not of steps taken per day. CONCLUSIONS In this study, the persons with obesity reported higher functional pain values during walking and stair climb and had lower lumbar strength compared with the overweight participants. Rehabilitation strategies that include lumbar extensor strengthening may help improve functional mobility and walking duration, both of which can help with weight management in older adults with obesity with chronic low-back pain.