Mindfulness techniques have a beneficial impact on mental health and everyday functioning, including reducing anxiety and stress and improving emotional regulation. Research methods that examine brain activity as well as functional and structural changes contribute to the understanding of the mechanisms behind mindfulness techniques, which are also applicable in sports. EEG studies and quantitative QEEG analysis, used to measure cortical activity, can be applied in assessing the effectiveness of various forms of mental training and their long-term effects on high-performance athletes. In the conducted experiment, a 19-point QEEG study showed changes in values of selected brain waves at specific points of the cerebral cortex, measured before and after completing a 7-day mindfulness meditation training. The presented case study of a swimmer from the Polish National Team (18 years old) shows changes in values of Sensorymotor Rhythm (SMR), Beta, and Beta2 waves in the temporal, frontal, and central areas of the brain’s cortex after the meditation training, which may have had a positive impact on better emotional regulation and stress management during the European Junior Championships. The presented training forms and measurements of their effectiveness can form the basis for preparing athletes for competitions. Further research in this direction is, however, necessary to confirm these observations.