Some of his highlights include: Known as "Lil' But Bad", Johnston turned pro in 1993 and fought for the WBC Lightweight Title against Jean Baptiste Mendy in 1997, winning a close split decision.

He defended the title four times, including a victory over Angel Manfredy, but lost the belt in a loss to José Luis Castillo in 2000 via majority decision.

The loss was declared the 2000 Upset of the Year by Ring Magazine, as Castillo was relatively unknown at the time and Johnston was thought to be over the hill.

Several minutes later it was discovered that Judge Ken Morita's scorecard (originally 115-114 Johnston) had been added incorrectly and should have read 114-114, thus making the bout a draw and allowing Castillo to retain his title.

He lost his next big fight, a WBC Lightweight Title Eliminator in 2003 to contender Juan Lazcano, a stunning 11th-round TKO loss.