The Philadelphia Phillies selected Frohwirth in the 13th round (335th overall) of the
1984 Major League Baseball draft. Frohwirth began the 1987 season in
Minor League Baseball (MiLB), with the
Double-A Eastern League (EL)
Reading Phillies and was leading the league with 16
saves when he was promoted to the
Triple-A Maine Guides of the
International League (IL), on June 29, 1987. On August 10, 1987, the Phillies promoted Frohwirth to the big leagues, when he made his MLB debut. Frohwirth entered the game in the fifth
inning, when Phillies teammate
Kevin Gross was caught with an illegal substance (sandpaper) in his glove, and was ejected. Frohwirth pitched an effective 1 innings and was awarded the
win in the team's 4–2 victory. Frohwith finished the 1987 minor league season with a 1.86
earned run average (ERA) with Reading and a 2.51 ERA with Maine, finishing the season with a combined 23 saves in MiLB. He won the 1987
Paul Owens Award as the top pitcher in the Phillies’
farm system. In 1988, Frohwith went back down to Maine, posting a 7–3 record, with a 2.44 ERA, and 13 saves, before returning to the Phillies, going 1–2 with an 8.25 ERA. He pitched infrequently enough to maintain his rookie status until 1989; that season, Frohwith had his breakout MLB season, as he appeared in 45 games, had a 3.59 ERA, and allowed fewer hits (56) than
innings pitched. After spending most of 1990 back at Triple-A, with the
Scranton/Wilkes-Barre Red Barons, Frohwirth was given his release by the Phillies. He then signed with the Baltimore Orioles and had three solid seasons, compiling a 2.71 ERA in 186 relief outings. Frohwirth also pitched for the Boston Red Sox and California Angels, before retiring following the 1996 campaign. ==Later career==