© 2019 The Obesity Society Objective: This study aimed to evaluate the impact of timing of exercise initiation on weight loss within a behavioral weight loss program. Methods: Adults with overweight or obesity (N = 170; age 18-55 years; BMI 25-42 kg/m2; 83.5% women) were enrolled in an 18-month behavioral weight loss program consisting of a reduced-calorie diet, exercise, and group-based support. The standard group (STD) received a supervised exercise program (progressing to 300 min/wk of moderate-intensity aerobic exercise) during months 0 to 6. The sequential group (SEQ) was asked to refrain from changing exercise during months 0 to 6 and received the supervised exercise program during months 7 to 12. On completion of supervised exercise, both groups were instructed to continue 300 min/wk of moderate-intensity exercise for the study duration. Results: At 6 months, the STD group exhibited greater reductions in body weight (−8.7 ± 0.7 kg) compared with the SEQ group (−6.9 ± 0.6 kg; P = 0.047). Between 6 and 18 months, the STD group regained more weight (2.5 ± 0.8 kg vs. 0.0 ± 0.8 kg; P = 0.02). At 18 months, there were no between-group differences in changes in weight (STD: −6.9 ± 1.2 kg; SEQ: −7.9 ± 1.2 kg), fat mass, lean mass, physical activity, or attrition. Conclusions: Both immediate and delayed exercise initiation within a behavioral weight loss program resulted in clinically meaningful weight loss at 18 months. Thus, timing of exercise initiation can be personalized based on patient preference.