Electroencephalogram (EEG) studies of mindfulness have shown it can lead to increases in alpha power, which are similar to those obtained by alpha-based neurofeedback (NF) interventions. It has been hypothesized there may be relationships between mindfulness and NF in terms of the neural pathways through which they induce salutary outcomes. The aim of the study was to evaluate possible changes in mindfulness and cognitive functioning following an alpha-based NF intervention, and the role of alpha power as a mediator of improvements. A controlled, non-randomized, trial with 50 healthy participants was conducted with two experimental conditions: a six-session NF intervention and a waiting-list control group. Both groups were administered mindfulness questionnaires (Mindful Attention Awareness Scale (MAAS), Five Facet Mindfulness Questionnaire (FFMQ)) and cognitive measures (Paced Auditory Serial Addition Task (PASAT)), at pre- and post-test. The NF intervention focused on the up-regulation of upper alpha power. Differences among groups were estimated using ANCOVAs, and mediation assessment through path analyses. Compared to controls, the NF group showed enhanced task-related upper alpha power (effect size (ES) = 1.16, p < 0.001), mindfulness outcomes (MAAS: ES = 0.94, p = 0.004; FFMQ: ES = 1.38, p < 0.001), and a trend of cognitive functioning (PASAT time: ES = 0.59, p = 0.062). Upper alpha power had a mediating effect for cognitive functioning (PASAT errors: indirect effect = 0.81, 95% CI = 0.21–1.85), but not for mindfulness. These results demonstrate the effectiveness of NF for increasing mindfulness in healthy individuals with no previous experience in mindfulness or neurofeedback training, suggesting that NF may be an acceptable method of augmenting mindfulness-related capacities in the general population.