Hermanson broke in with the Padres in 1995, but pitched sparingly in his two seasons in San Diego. He was sent to Florida in November 1996 for Quilvio Veras and was passed along to Montreal for Cliff Floyd just four months later.
Armed with a low-90s fastball, a slider, and a changeup, Hermanson continued to show promise in 1997, and after a 14-11 season with a 3.13 ERA in 1998 the 25-year-old was the ace of the Montreal Expos‘ rotation and on the verge of baseball stardom. But in 1999 he lost command of his pitches and although he chewed up 216 innings for the Expos, he posted a 4.20 ERA.
The next year Hermanson’s ERA continued to rise. With injuries to Scott Strickland and Ugueth Urbina, manager Felipe Alou decided to try him as a closer, but Hermanson blew three saves in seven opportunities in May 2000 and was quickly returned to the rotation.
The Expos finally gave up on him and traded him to the St. Louis Cardinals with Steve Kline for Fernando Tatis and Britt Reames after the season. The Cardinals hoped a change of venue would rescue his career and Hermanson responded with a 4.45 ERA and a 14-13 record.