Mustain was highly recruited out of high school. He was named the High School Player of the year by PARADE (January 2006), 2005-2006 Gatorade National Player of the Year, and the 2005 USA Today National Player of the Year. Scout.com ranked him as the top high school recruit for the class of 2006. After the University of Arkansas's decision to hire Mustain's then high school football head coach Gus Malzahn as an offensive coordinator, Mustain officially committed to the University of Arkansas. Malzahn's hire was widely viewed as an attempt to recruit Mustain.
University of Southern California (2007–2010)
In 2007, Mustain transferred to the University of Southern California (USC). Under NCAA transfer rules, Mustain sat out the 2007 season and served as the quarterback for the scout team. After regaining playing eligibility in 2008, he alternated between the second- and third-string quarterback positions behind starters Mark Sanchez and Matt Barkley. At USC, Mustain completed 52-for-89 passes for 505 yards with three touchdowns and three interceptions.
Taking a break from football, Mustain signed a minor league baseball contract with the Chicago White Sox's Bristol White Sox. Mustain made eleven appearances with Bristol before moving on to the Kannapolis Intimidators, where he pitched in eight games. Overall, he pitched in 19 games in 2012, going 2-2 with a 4.63 ERA, and one save.