He played in Major League Baseball as a left-handed pitcher between 1965 and 1972 for the St. Louis Cardinals, Montreal Expos and the Atlanta Braves.

Before Jaster was signed as an undrafted free agent by the St. Louis Cardinals on January 1, 1962, he attended Northwood University.

Jaster spent most of 1966 in the big leagues where he finished with a record of 11–5 with an ERA of 3.26 which earned him a tie for 4th in the Rookie of the Year balloting for that season.

The main highlight of Jaster's 1966 season, were his league leading five shutouts, all of which were against the NL pennant winning Los Angeles Dodgers.

He made history on April 14, 1969 by throwing the first pitch in a Major League regular season game in Canada.

After the 1969 season was over, Jaster was sent to the Atlanta Braves in exchange for pitcher Jim Britton and minor league catcher Don Johnson.