Using the test scores of more than 2,250,000 students who participated in the American Mathematics Competitions (AMC) from 2009 to 2019, this study examined the direction and magnitude of the trends in gender disparity in participation and high achievement in self-select mathematics exams. The results of this study indicated that the male to female ratio among the AMC participants increased significantly from 2009 to 2019. The findings also showed an established disparity that favored males for each year and for all competitions in both the top 1% and 5% levels, while the trend in the male to female ratios over a decade was stable, except for the top 1% of the population in the AMC 8, in which there was a significant increasing trend that favored males. The possible impacts of these findings within the context of the underrepresentation of women in STEM (science, technology, engineering, and mathematics) fields are discussed.