The aim of this study was to examine whether treatments based on different theories change pain catastrophizing and internal control of pain, and whether changes in these factors mediate treatment outcome. Participants were 211 patients with nonspecific chronic low back pain (CLBP) participating in a randomized controlled trial, attending active physical treatment (APT, n = 52), cognitive-behavioral treatment (CBT, n = 55), treatment combining the APT and CBT (CT, n = 55), or waiting list (WL, n = 49). Pain catastrophizing decreased in all 3 active treatment groups and not in the WL. There was no difference in the change in internal control across all 4 groups. In all the active treatment groups, patients improved regarding perceived disability, main complaints, and current pain at post-treatment, and no changes were observed in the WL group. Depression only changed significantly in the APT group. Change in pain catastrophizing mediated the reduction of disability, main complaints, and pain intensity. In the APT condition, pain catastrophizing also mediated the reduction of depression. Not only cognitive-behavioral treatments but also a physical treatment produced changes in pain catastrophizing that seemed to mediate the outcome of the treatment significantly. The implications and limitations of these results are discussed. Perspective: This article shows that treatment elements that do not deliberately target cognitive factors can reduce pain catastrophizing. Reduction in pain catastrophizing seemed to mediate the improvement of functioning in patients with chronic low back pain. The results might contribute to the development of more effective interventions.