Given changes in technology, regulatory guidance, and COVID-19, there has been an explosion in the number of online studies in the social and clinical sciences. This, in turn, has led to a need for brief and accessible instruments that are designed and characterized with self- administered, online research in mind. To fulfill this need, the Brief Attention and Mood Scale of 7 Items (BAMS-7) was developed and validated in five cohorts and four experiments to assess real-world attention and mood. In Experiment 1, an exploratory factor analysis was run on data from a large, healthy, adult sample (N=75,019, ages 18-89 years). Two subscales were defined and further characterized: one for Attention, the other for Mood. In Experiment 2, convergent validity (concordance) with existing questionnaires was established in a separate sample (N=150). Experiment 3 used a receiver-operating characteristic (ROC) analysis to demonstrate known- groups validity of the Attention and Mood subscales using a large sample (N=58,411) of participants reporting a lifetime diagnosis of ADHD, anxiety, or depression, as well as the healthy sample from Experiment 1. Experiment 3 also showed that the BAMS-7 Attention subscale provided superior classification performance for ADHD, and the Mood subscale provided superior classification for anxiety and depression. Finally, Experiment 4 applied the BAMS-7 definition to reanalyze data (N=3,489) from a previously published cognitive training study (Hardy et al., 2015), finding that the Attention and Mood subscales were sensitive to the intervention (compared to an active control) to different degrees. In sum, the elucidated psychometric properties and large normative dataset (N=75,019) for BAMS-7 may make it a useful instrument for assessing real-world attention and mood.