This research aims to evaluate the effect of the MindKinder adult program (MK-A) on the variables of life satisfaction and positive and negative affect. We also intend to analyse the differential effects in the experimental group depending on the level of previous emotional well-being and previous trait emotional intelligence. A total of 137 university students (pre-service teachers) in their first year of the Education degree program at a university participated in the study. Participants were randomly assigned to the experimental group (n = 72) and the control group (n = 65) using a cluster-randomised controlled trial. The results showed significant improvements in the experimental group compared to the control group in terms of subjective well-being, positive affect, and negative affect. Regarding the experimental group’s results, based on previous emotional well-being and trait emotional intelligence scores, the findings suggest that individuals with greater psychological vulnerability may benefit more from interventions like these.