Mills' primary objective as president sought to prevent players from switching from one professional league to the next, which they typically did during mid-season in search of higher salaries. In 1888, Mills called an assembly of representatives from the three professional leagues—the National League, American Association, and Northwestern League—in what was dubbed as the "Harmony Conference."
Mills' goal was to stop players from switching leagues during mid-season for better pay. In 1888, he organized a meeting of representatives from the three major professional baseball leagues—the National League, American Association, and Northwestern League—known as the "Harmony Conference."