The abilities to stabilize the focus of attention, notice attention lapses, and return attention to an intended object following lapses are precursors for mindfulness. Individuals diagnosed with attention-deficit hyperactivity disorder (ADHD) are deficient in the attentional and self-control skills that characterize mindfulness. The present study assessed the relationship between mindfulness and ADHD in young adults using the Mindful Attention and Awareness Scale (MAAS), a computerized Go/No-Go task (the Test of Variables of Attention (TOVA)), the World Health Organization Adult Self-Report Scale (ASRS), a tool used as an adult ADHD screen, the Beck Anxiety Inventory (BAI), and the Beck Depression Inventory-II (BDI-II). We recruited 151 adult volunteers (ages 18 to 40); 100 with confirmed ADHD diagnoses and 51 control participants. Overall, participants with prior diagnoses of ADHD scored lower on the MAAS than controls and ASRS scores were strongly negatively correlated MAAS scores. Attention performance index, response time, and response-time variability subscales of the TOVA were positively correlated with MAAS scores and negatively correlated with ASRS scores. Intrasubject response-time variability on the TOVA, a parameter associated with attention lapses, was also strongly negatively correlated with MAAS scores. Overall, participants' self-reported mindfulness, as measured by the MAAS, was strongly related to self-reports on a clinical measure of attention disorders, anxiety, depression, and multiple indices of concentration and mind wandering on a standardized Go/No-Go task, the TOVA.