This study utilized ecological momentary assessment (EMA) to sharpen understanding of whether non-suicidal self-injury (NSSI) thought intensity and duration impacts NSSI behavior and potential alternative behaviors to NSSI. Self-injuring adolescents and young adults (N = 47) were randomly prompted using EMA to complete measures of NSSI thoughts (i.e., occurrence, intensity, duration) and behavior (i.e., engagement, frequency, duration, and methods) for two weeks. The most common NSSI thought duration was 1–30 min. Individuals with higher NSSI thought intensity were more likely to self-injure. NSSI thought intensity and duration interacted to predict NSSI frequency, duration, the number of NSSI methods, and the likelihood of cutting such that the positive relationship between NSSI thought intensity and these outcomes became stronger at longer NSSI thought durations. The most commonly endorsed non-NSSI behaviors following NSSI thoughts were pushing thoughts away, talking to others, and distraction (e.g., going out). Participants rarely endorsed using social media or online NSSI support groups in lieu of NSSI. Resisting intense NSSI thoughts over prolonged time periods may deplete the self-regulatory resources required to terminate NSSI episodes once they have begun, leading to greater NSSI frequency, longer NSSI duration, and more NSSI methods. Bolstering social support may help to reduce NSSI.