Despite being a first-line treatment recommendation, there is uncertainly for how exercise helps people with chronic low back pain. We designed this study to examine how exercise might help people with chronic low back pain by following a large community sample for 1-year. Qualitative questionnaires and self-report measures were collected every 3-months for 1-year in 400 people with chronic low back pain. People were not provided any specific treatment advice as part of this study but were allowed to engage with any normal physical activity, treatment, or medication as part of their normal life. Exercise engagement was defined from inspection of participant qualitative responses, according to minimum acceptable levels of exercise that elicit symptom reduction. Multiple mediation analysis was performed to examine the effect of exercise engagement on disability through the proposed mediators (pain, fear, catastrophizing, depression, anxiety, self-efficacy). The significant effect of exercise engagement on reductions in disability at 6- and 12-months was explained through pain and catastrophizing. People with chronic low back pain who reported worsening of symptoms over the year had similar reporting of exercise throughout the 12-months to people who had improvements in disability. Exercise can reduce disability through the effect on pain and catastrophizing, but how this effect occurs (i.e., an active or passive component of exercise) is unclear.