Many psychological constructs under the heading of Positive Psychology are important to the careers of athletes and other high level performers. Grit and optimism have both been linked to performance outcomes in multiple disciplines, and interventions have been trialed to develop these factors in individuals and groups, with varying levels of success. Using an adapted version of the Penn Resiliency Program (PRP) and Master Resilience Training (MRT) programs used to train resilience in both education and military settings, we implemented a mental toughness training intervention with a group of professional football players (n = 22) and also followed a control group (n = 13) in the same team. Results show the training was effective at increasing optimism but not grit, and had a positive impact on performance over the following season that was moderated by previous performance levels. Correlations and regression models also showed that optimism and grit together were subsequently predictive of consistency and performance. We discuss the moderating effects of the player’s pre-training level of ability as well as potential leadership effects, and offer recommendations for future research into the links between optimism, grit and performance.