During that time, he pitched for the California Angels, Minnesota Twins, Baltimore Orioles, and Milwaukee Brewers of the American League (AL).

In 1975, he posted one of the best seasons in Lehigh history for a pitcher, setting school records with 92 innings, 10 complete games, and a 1.66 earned run average (ERA).

On Sunday June 26, 1977, Hartzell became one of the very few big league pitchers to be credited with two wins in one day, by beating the Texas Rangers in both ends of a doubleheader.

That year, the Angels challenged for the West Division title, finishing with a record of 87-75, five games behind the Royals.

On February 3, 1979, the Angels traded Hartzell, along with Dave Engle, Brad Havens, and Ken Landreaux, to the Twins for future-Hall of Famer, Rod Carew.

Hartzell was the only pitcher to beat the AL-Champion Orioles twice during the regular season and his 2.4 walks-per-nine innings was seventh best in the league.

After Hartzell was released by the O’s, he made several not-so-successful comeback attempts with Milwaukee, only pitching another 10.1 innings in MLB, thereafter, for the 1984 Brewers.