1 American League and National League were joined into Major League Baseball in 1999. Mission: to 'unify the leagues' 2 First Commissioner was Bug Selig. Appointed in 1999. 3 A. Bartlett Giamatti, who was also a National League president (1986-1989), and was the second and most recent league president to become Commissioner (Apr 1, 1989 through Sep 1, 1989). 4 Several other professional baseball leagues: American Association, Federal League, National Association, Players League, Union Association.