Dishion and Patterson's work on the unique role of fathers in the coercive family process showed that fathers' coercion explained twice the variance of mothers' in predicting children's antisocial behavior and how treatment and prevention of coercion and promotion of prosocial parenting can mitigate children's problem behaviors. Using these ideas, we employed a sample of 426 divorced or separated fathers randomly assigned to Fathering Through Change (FTC), an interactive online behavioral parent training program or to a waitlist control. Participating fathers had been separated or divorced within the past 24 months with children ages 4 to 12 years. We tested an intent to treat (ITT) mediation hypothesis positing that intervention-induced changes in child problem behaviors would be mediated by changes in fathers' coercive parenting. We also tested complier average causal effects (CACE) models to estimate intervention effects, accounting for compliers and noncompliers in the treatment group and would-be compliers in the controls. Mediation was supported. ITT analyses showed the FTC obtained a small direct effect on father-reported pre–post changes in child adjustment problems (d = .20), a medium effect on pre–post changes in fathers' coercive parenting (d = .61), and a moderate indirect effect to changes in child adjustment (d = .30). Larger effects were observed in CACE analyses.