Training in emotional regulation skills is one of the most important resources for the adaptation of athletes to contexts of sports pressure, especially during competitions. This study explored the effects of a mindfulness programme (Flow Meditation) on levels of impulsivity, mood and pre-competition anxiety-state in a sample of athletes (N = 41, 22.83 ± 5.62 years). Participants were randomly assigned to an intervention group (N = 21; 14 males and 7 females) which received the intervention over 10 weeks (a weekly session) and a control group (wait-list; N = 20; 13 males and 7 females). The variables under study were assessed through different questionnaires at pre- and post-test (T1–T2) in both groups. The mindfulness intervention was effective in reducing impulsivity (cognitive (t = −4.48, p ≤ 0.001, Cohen’s d = 1.40), both motor (t = −4.03, p ≤ 0.001, Cohen’s d = 1.20) and unplanned (t = −5.32, p ≤ 0.001, Cohen’s d = 1.66)), mood (tension (t = −4.40, p ≤ 0.001, Cohen’s d = 1.37), depression (t = −4.56, p ≤ 0.001, Cohen’s d = 1.42), anger (t = −7.80, p ≤ 0.001, Cohen’s d = 2.47), somatic anxiety (t = −5.28, p ≤ 0.001, Cohen’s d = 1.65), and cognitive anxiety (t = −6.62, p ≤ 0.001, Cohen’s d = 2.07) in the intervention group compared to the control group and with large to very large effect sizes. Mindfulness is a factor that enhances athletes’ ability to cope with high sport pressure and the healthy management of competition (e.g., fear of failure), or with their daily life.