Johnston was drafted in the 3rd round of the 1988 amateur entry draft by the Royals. Less than 5 seasons later, on September 5, 1991, he made his major league debut at the age of 24. Perhaps his rookie season was his best season—in just over 22 innings, he gave up only one earned run for an ERA of 0.40. He also struck out 21 batters that year.
According to Baseball America, in 1992 Johnston was the Royals top prospect, and was also number 59 on Baseball America's 1992 Top 100 Prospects list.
After his promising rookie season, Johnston pitched in just 5 games in 1992, posting an ERA of 13.50. After the season Johnston was traded to the Pirates with pitcher Dennis Moeller for second baseman José Lind, and he rebounded to an ERA of 3.38 in 33 games. However, after brief trials producing poor ERAs of 29.70 (1994) and 11.25 (1995), Johnston pitched his final major league game on May 12, 1995.
