A 13-1 record with a 1.11 ERA in Single-A in 1985 earned Mathews a midseason promotion to Triple-A Louisville. His eleven victories in 1986 were the most by a St. Louis Cardinal rookie pitcher in fifteen years. He spent half of the 1988 season disabled with a strained quadricep; more injuries caused him to miss all of 1989.
Mathews returned in 1990, but failed to earn a win in ten starts and became a free agent after refusing a minor-league assignment. He resurfaced with the Philadelphia Phillies in 1992, but went just 2-3 in 14 games.