Sport participation has been shown to be associated with health and social benefits. However, there are persisting inequities and barriers to sport participation that can prevent children and young people with diverse backgrounds and abilities from accessing these benefits. This mixed methods study investigated how diversity is understood, experienced and managed in junior sport. The study combined in-depth interviews (n = 101), surveys (n = 450) and observations over a three-year period. The results revealed that a focus on performance and competitiveness negatively affected junior sports clubs' commitment to diversity and inclusive participation. Gender and a range of attitudes about diversity were also strongly related. On average, we found that those who identified as men were more likely to support a pro-performance stance, be homophobic, endorse stricter gender roles, and endorse violence as a natural masculine trait. In addition, those who identified as men were less likely to hold pro-disability attitudes. These findings suggest that the participation-performance tension and gender affect to what extent, and how, sports clubs engage children and young people with diverse backgrounds and abilities.