Global Range: Native to most of south-central U.S., north to Indiana and Illinois, west to Texas, south to southern Mexico, east to Mobile River system. Occurs also in the drainages of the Chattahoochee and Savannah rivers (Lydeard and Wooten 1991). See Walters and Freeman (2000) for information on the distribution of G. affinis and G. holbrooki in the Conasauga River system, where G. affinisis widespread and native and G. holbrooki is apparently introduced and expanding its range. Widely introduced in the western United States and throughout the world.

Lynch (1992) reported that five or six populations from Georgia, Illinois, Tennessee and Texas were used for most introductions nationwide and worldwide. Within the United States, sources from Illinois, Tennessee and Texas were used to establish mosquitofish in the western half of the country. Therefore, most if not all populations in the western United States are G. affinis.