Procrastination is a common self-regulatory failure that can have a negative impact on well-being and performance. However, few clinical trials have been conducted, and no follow-up has ever been performed. The current study therefore aimed to provide evidence for the long-term benefits and investigate predictors of a positive treatment outcome among patients receiving Internet-based cognitive behavior therapy (ICBT). A total of 150 self-recruited participants were randomized to guided or unguided ICBT. Self-report measures of procrastination, depression, anxiety, and quality of life were distributed at pre-treatment assessment, post-treatment assessment, and one-year follow-up. Mixed effects models were used to investigate the long-term gains, and multiple linear regression for predictors of a positive treatment outcome, using the change score on the Irrational Procrastination Scale as the dependent variable. Intention-to-treat was implemented for all statistical analyses. Large within-group effect sizes for guided and unguided ICBT, Cohen’s d = .97–1.64, were found for self-report measures of procrastination, together with d = .56–.66 for depression and anxiety. Gains were maintained, and, in some cases, improved at follow-up. Guided and unguided ICBT did not differ from each other, mean differences −.31–1.17, 95% CIs [−2.59–3.22], and none of the predictors were associated with a better result, bs −1.45–1.61, 95% CIs [−3.14–4.26]. In sum, ICBT could be useful and beneficial in relation to managing procrastination, yielding great benefits up to one year after the treatment period has ended, with comparable results between guided and unguided ICBT.