Purpose: The quality of life is compromised in children and adolescents with overweight or obesity. The aim of this study was to evaluate whether the quality of life improves during a community-based overweight and obesity treatment, and whether improvements depend on reductions in the degree of obesity. Methods: Quality of life was assessed using the Pediatric Quality of Life Inventory (PedsQL) 4.0 in children and adolescents aged 3–18 years with overweight or obesity [body mass index (BMI) ≥85th percentile] upon entry into a community-based chronic care overweight and obesity treatment based upon The Children’s Obesity Clinic’s Treatment protocol, and upon follow-up after 10–30 months of treatment. Height and weight were measured at each consultation and converted into a BMI standard deviation score (SDS). Results: Upon entry, 477 children (212 boys) completed a PedsQL, and 317 (143 boys) completed another PedsQL after a median of 13 months of treatment. Quality of life improved (p < 0.001), regardless of sex, age, and pubertal development stage upon entry (p ≥ 0.108). Greater reductions in BMI SDS and high socioeconomic status were associated with greater improvements in the quality of life (p ≤ 0.047). However, improvements also occurred in children and adolescents with low socioeconomic status or who increased their BMI SDS (p < 0.001). Conclusions: Improvements in quality of life occurred in children and adolescents during a community-based overweight and obesity treatment, even in children and adolescents who increased their BMI SDS. Thus, improvements may be due to the treatment itself and not exclusively to reductions in BMI SDS. Trial registration: Clinicaltrials.gov, ID-no.: NCT02013843.