After earning All-America honors and being named Cal-Hi Sports state player of the year at Clovis High, the right-hander became one of the greatest pitchers in NCAA history at Fresno State and a World Championship gold-medal winner for Team USA. Southern excelled in the circle and in the classroom for the Bulldogs as a four-time All-American and two time Academic All-American. Upon ending her collegiate career in 2005, she ranked among the Top 10 in nine NCAA categories and in the Top 10 among 18 Fresno State pitching and hitting categories. She was named to the ESPN Rise All-Decade softball team in 2009. Southern accomplished this while overcoming a major shoulder injury four games into her freshman season and arm trouble her junior year.

With the Bulldogs, she posted an 118-34 record, with 79 shutouts, eight no-hitters, four perfect games, 0.51 ERA, and 1,321 strikeouts in 1,088 ⅔ innings. At the plate, she hit .314, with 29 home runs, 127 RBI and a slugging percentage of .479 over her career. Among other collegiate honors: 2005 Western Athletic Conference Female Athlete of the Year, four-time WAC Pitcher of the Year, four-time All-WAC, two-time Fresno State Female Athlete of the Year, and two-time NCAA earned-run average leader.

Southern helped the Fresno Force win the under-16 Amateur Softball Association national championship in 1997 and was named an ASA All-American in 1998 and 1999. Following her final season at Fresno State, she was a first-round professional draft pick and pitched for two championship teams in the National Pro Fastpitch League (2005, 2008). Her bid for a spot on the U.S. 2008 Olympic team was ended by a neck injury. She has coached at the high school and college levels.