Youth with intellectual disabilities engage in low levels of physical activity (PA). An aim of this family-based weight-loss behavioral intervention (FBBI) trial was to increase and sustain PA in these youth. Accelerometry data were available from 21 individuals with intellectual disabilities, age 14–22 years. Each completed the 6-month FBBI, after which 10 completed a 6-month maintenance intervention (FBBI-M), and 11 received no further intervention (FBBI-C). Twenty participated in a further 6-month follow-up. Accelerometry data were analyzed using linear mixed models. During FBBI, mean (SE) moderate to vigorous PA increased by 4.1 (2.5) min/day and light PA by 24.2 (13.5) min/day. Mean (SE) difference in moderate to vigorous PA between participants in FBBI-M and FBBI-C at 18 months was 14.0 (5.1) min/day (p = .005); mean (SE) difference in light PA was 47.4 (27.4) min/day (p = .08). Increasing PA through behavioral intervention is possible in youth with intellectual disabilities.