He played college football for the Gardner–Webb Runnin' Bulldogs and was selected by the Kansas City Chiefs in the seventh round of the 2008 NFL draft.
Johnston was a defensive end at Madison High School in San Diego, California, where he was a two-time First-team All-Harbor League selection (as a junior and senior).
In 2006, he started 11 contests at left defensive end, totaling 77 tackles (35 solo), 14 tackles for loss, 8 sacks, two forced fumbles and one fumble recovery and was Second-team All-America honors by The NFL Draft Report, Associated Press and Football Gazette and was named the conference's Defensive Player of the Year and a First-team All-Big South Conference selection.
In 2005, he started all 11 games in his first season at right defensive end, totaling 59 tackles (28 solo) with 5 sacks and 12.5 tackles for loss, four forced fumbles, one fumble recovery, one pass defensed and a blocked kick and was also named First-team All-Big Conference for the first time.
[5] After spending the 2009 season out of football, Johnston signed a future contract with the Miami Dolphins on January 6, 2010.