Biased thinking is a common feature of patients presenting with borderline personality disorder (BPD). For the treatment of BPD, it was shown that the individualizing of the treatment, by using the motive-oriented therapeutic relationship (MOTR), had a beneficial short-term effect on process and outcome. So far, it remains unclear what the role of early change in biased thinking is in these treatments. The present study aims to assess whether there is a link between the MOTR, change in biased thinking, and outcome. The sample (N = 60) is based on a randomized controlled trial with two conditions: (a) 30 patients in a 10-session version of psychiatric management, and (b) 30 patients in a 10-session version of psychiatric management augmented with the MOTR. For each patient, three sessions (intake, middle, late) were selected, transcribed, and rated using the Cognitive Errors Rating Scale (CERS). An overall decrease of negative cognitive errors during 10 sessions of treatment was observed, independently of the treatment condition. No specific effect related to change in biased thinking may be attributed to the individualizing of the treatment. These results are discussed with regard to mechanisms of change in treatments for BPD, in particular with regard to the central role that biased thinking, as well as the MOTR, might play early in treatment.