Cognitive-Behavioural Therapy (CBT) is an evidence-based treatment for Generalized Anxiety Disorder (GAD). Worry postponement (WP), in which a client is asked to postpone worry until a 30-minute “worry time,” is a common component of CBT for GAD; however, the efficacy of WP has never been tested in people with GAD. Further, the mechanisms of change of WP are not known; nor are its effects on cognitive processes and symptoms related to GAD. A better understanding of the efficacy and mechanisms of change of WP could help to optimize CBT for GAD. The goals of the present study were to examine, in a sample of people with GAD, the effects of WP on worry and GAD symptoms, and cognitive processes and symptoms related to GAD. The study also examined the effects of WP on two proposed mediators: stimulus control and metacognitive beliefs. Sixty-seven adults were randomized to one of three conditions: 2- week worry postponement intervention (WP), 2-week worry monitoring intervention (MON), or an assessment only control. Participants completed outcome measures before and after the 2- week intervention period and at a 2-week follow-up. In the WP and MON conditions, participants completed daily worry monitoring using a phone-based application. All participants showed a significant decrease in past-week worry over the course of the study, with no significant differences between the conditions. There were no significant changes in GAD symptoms across conditions. There was no evidence that WP had superior effects to control groups on cognitive processes or symptoms related to GAD. There was no evidence that stimulus control or metacognitive beliefs mediated the reduction in past week worry in WP. This is the first known study to examine the effects of WP in people with GAD. Whereas worry did decrease on some indices over the course of the study, there were no significant differences between WP and two control conditions. Further this study found no evidence that WP has specific effects on two processes that are thought to be mechanisms of action. The findings of this study demonstrate the need to establish the efficacy of the treatment components used in CBT.