Jiménez' career in MLB commenced as a starting pitcher with the Cardinals and he converted to relief pitching with the Rockies, saving more than 100 games.

As a rookie with the Cardinals in 1999, Jiménez pitched a no-hitter in the first of two consecutive starts defeating the Arizona Diamondbacks and future Hall of Famer Randy Johnson in 1−0 complete game shutouts.

After converting to relief with the Rockies, he set club records for saves in a single season with 41 in 2002 and career with 102.

In Jiménez' second start after the no-hitter, he again faced Johnson and the Diamondbacks, winning a second consecutive complete game shutout decision by a 1−0 score while allowing two hits.

The following season he swung back into a starter, making seven starts and 20 saves for the Rockies, but ballooned his ERA to 5.22 and again having a record of 2–10.