Mental strength is defined as a person’s ability to persist through challenging situations and recover from hardships and failures. Due to combat sports (CS) vigorous nature, several authors have identified various psychological factors modified through CS engagement and experience. This research aimed to determine the psychometrics of the Mental Strength Scale, explore the correlations between mental strength and age, CS experience, and competitive engagement, and investigate the existing group differences in mental strength based on sex, age, CS engagement, competition engagement, and CS experience. The total sample included 431 participants from 18 to 67 years of age, including 373 CS practitioners and 58 non-practitioners. Results revealed that The Mental Strength Scale demonstrated good internal validity (Cronbach’s alpha = 0.809) and convergent validity (Grit Scale correlation: p < .001, r = .539; Brief Resilience Scale correlation: p < .001, r = .551). A positive correlation was found between CS experience and mental strength. Group comparison demonstrated higher mental strength in CS practitioners than non-practitioners and in men compared to women. Moreover, CS participants in the >5, 2-5, and <2 years of experience reported higher mental strength than participants with no CS experience. In conclusion, the findings confirm that the Mental Strength Scale is a good psychometric instrument to assess the degree of mental strength, particularly in CS practitioners, and that CS engagement and experience positively influence the development of mental strength.