The conceptual overlap between mind-wandering and attention-deficit/hyperactivity disorder (ADHD)-related impairments is considerable, yet little experimental research examining this overlap among children is available. The current study aims to experimentally manipulate mind-wandering among children with and without ADHD and examine effects on task performance. Participants were 59 children with ADHD and 55 age-matched controls. Participants completed a novel mind-wandering sustained attention to response task (SART) that included non-self-referential and self-referential stimuli to experimentally increase self-referential mind-wandering, reflected by increases in reaction time variability (RTV) following self-referential stimuli. The ADHD group participated in a classroom study with analogue conditions aimed at encouraging self-referential future-oriented thinking (free play/movie before and after class work) compared to a control condition (newscast) and a cross-over methylphenidate trial. The significant interaction between ADHD status and self-referential stimuli on SART performance indicated that self-referential stimuli led to greater RTV among children with ADHD (within-subject d = 1.29) but not among controls. Methylphenidate significantly reduced RTV among youth with ADHD across self-referential (d = 1.07) and non-self-referential conditions (d = 0.72). In the ADHD classroom study, the significant interaction between mind-wandering condition and methylphenidate indicated that methylphenidate led to higher work completion (ds > 5.00), and the free-play mind-wandering condition had more consistent detrimental effects on productivity (ds ≥ 1.25) than the movie mind-wandering condition. This study is the first to manipulate mind-wandering and assess effects among children with ADHD using a behavioral task. Results provide evidence that children with ADHD are uniquely susceptible to mind-wandering interference.