The recent COVID-19 pandemic has severely impacted the mental health of nurses caring for institutionalized older people. Caring in this environment can be complex, with higher levels of burnout and compassion fatigue in staff. It is therefore important to find interventions to increase the well-being of staff. Mindfulness training is known to be effective in treating a variety of physical and mental health conditions. This study sought to conduct a direct evaluation of the effectiveness of a combined online training in two types of mindfulness-based therapies for the reduction of compassion fatigue and burnout in geriatric nurses caring for institutionalized elderly people with dementia. In a randomized controlled trial (n = 39 experimental group, n = 35 control group), we explored whether individuals with high levels of burnout and compassion fatigue would benefit more from an online mindfulness training program. The outcome variable was the ProQoL professional quality of life scale, which was collected at baseline, at six weeks, and at three months after completion of the intervention. Significant differences were found between both groups for the subscales Compassion Fatigue and Burnout (p < 0.05), with a significant improvement in the experimental group (significant effect size). These findings were maintained at three months after the end of the intervention for both compassion fatigue (F1,28 = 18.14, p = 0.003) and burnout (F1,28 = 7.25, p = 0.040). However, there were no differences between groups for the satisfaction subscale. The effect of time and the effects of comparing the two groups after controlling for time were statistically significant for all three subscales of the questionnaire (all p values < 0.001), with effect sizes ranging from small to large (R2 change 0.10–0.47). These data indicate that the experimental condition was more effective, explaining between 10 and 18% more of the variance. A short, online intervention based on mindfulness training appears to be effective for reducing compassion fatigue and burnout in geriatric nurses, with sustained effects over time.