Called up to the Cardinals late in 1965, Jaster threw three complete-game victories in three starts. In 1966 he threw five shutouts, all against the pennant-bound Dodgers, to set a major league record for most consecutive shutouts won from one club in a season. He finished the year with a career-high 11 victories. Relieving in the third inning of Game Six of the 1968 World Series, Jaster pitched to three batters without retiring any, surrendering a grand slam to Jim Northrup, as the Tigers scored a Series-record 10 runs. He threw the first ML pitch in Canada, pitching for the Expos in their 1969 home opener.