Background: A poor understanding of the specific lifestyle behaviors that result in weight loss has hindered the development of effective interventions. The aim of this paper was to identify potential behavioral mediators of weight loss in the Healthy Dads, Healthy Kids (HDHK) intervention for overweight fathers. Findings: The three-month intervention was evaluated in a randomized controlled trial and conducted in Newcastle, New South Wales, Australia. Baseline, three month (immediate post-intervention) and six month assessments were conducted. Recruitment and follow-up occurred between October 2008 and May 2009. The study sample included 53 overweight/obese men [mean (± SD) age = 40.6(± 97.1) years; body mass index (BMI) = 33.2 (±3.9) kgm-2] and their primary school-aged children [n = 71, 54% boys; age = 8.2 (±2.0) years] who were randomized to HDHK program or a wait-list control group. Physical activity (PA) was assessed using pedometers and dietary behaviors were measured using a validated food frequency questionnaire. The intervention resulted in significant weight loss (−5.13 ± 1.27 kg, P<0.0001) and increased PA among fathers (2769 ± 750 steps/day, P<0.001) and their children (1486 ± 521 steps/day, P<0.01). Fathers’ PA mediated weight loss in the intervention (AB = −2.31, 95% CI = −4.63 to −0.67) and was responsible for 47% of the intervention effect. Changes in dietary behaviors were not statistically significant. Conclusions: PA was an important mediator of weight loss in the HDHK intervention. Encouraging overweight fathers to be more active with their children appears to be a promising strategy for obesity treatment in men.