Studies evaluating the benefit of adding motivational interviewing (MI) to behavioural weight loss programs (BWLPs) have yielded mixed findings. The aims of this randomized controlled trial were to: (1) assess the efficacy of adding MI to a BWLP on weight loss and adherence among 135 overweight and obese individuals (77.8% female; mean BMI = 33.6 kg/m2) enrolled in a 12-week BWLP, and (2) explore levels of importance, confidence, and readiness for change ratings. Participants, who were randomized to receive 2 MI sessions or 2 attention control sessions, were assessed at baseline, end of BWLP, and 6 months post-BWLP. Both groups decreased their weight from baseline to end of the BWLP; however, there was no weight change from baseline to 6 months post-BWLP in either group. We observed no group differences in importance, confidence, and readiness for change after each session. Participants may not have benefited from MI because they were already highly motivated to change. These findings suggest that pre-treatment assessment and treatment monitoring may help enhance MI+BWLP efficacy by guiding a stepped-care approach that identifies individuals for whom additional MI sessions are needed, and when. A focus on refining elements of treatment remains an important direction for effective obesity treatment.