Johnston began running in 1986 at a 5K run in Portland, Oregon. During her career, Johnston ran in various lengths of marathons ranging from 3 kilometre runs to full marathons. Her first win in a full marathon came at the 1993 Houston Marathon with a time of 2:29:05. The following year, Johnston won her only World Marathon Majors at the 1994 Chicago Marathon with a time of 2:31:34. Before competing at the 1995 Chicago Marathon, Johnston told the Daily Herald Suburban Chicago that she had "almost quit running altogether before last year's Chicago marathon" due to a long term back injury.
