After graduating from Poteet, he attended Baylor University where he played baseball from 1997 to 1999. Following his junior season at Baylor, Jennings was named by both Baseball America and Collegiate Baseball as the 1999 National Player of the Year. In what would be his final season at Baylor, he struck out 172 hitters in 146.2 innings and hit .382 with 17 homers and 68 RBIs. He also won the Golden Spikes Award, his second consecutive Big 12 Player of the Year, consensus All-America honors, the Outstanding Player on the 1999 Big 12 All Tournament Team, and a spot on the 1999 All-Big 12 Academic First Team.
From 2003 to 2005, Jennings had three losing seasons and posted an ERA above 5 each year. In 2006, Jennings pitched much better, ending the season with a 3.78 ERA, but had only a 9-13 record due to a lack of run support that ranked near the bottom of the league. From May 29 to the end of the season, Jennings had a 3.17 ERA, which was 4th in the Majors and 2nd in the NL behind Roy Oswalt. He also finished with over 200 innings pitched. He ended his career with the Rockies as the franchise's all-time winningest pitcher. He was surpassed in wins by a Rockies pitcher on June 23, 2009 by former teammate Aaron Cook.
2007 was a lackluster year with the Astros for Jennings. In a game against the Padres, he gave up 11 earned runs on 8 hits in two-thirds of an inning with three walks and no strikeouts.
On January 17, 2008, he signed a one-year contract with the Texas Rangers. Jennings ended up going on the DL after just six starts, all in April (missing the rest of the season), in which he struggled mightily. He went 0-5 with an 8.56 ERA, allowing eight home runs in 27⅓ innings of work, and he made it past the fifth inning only once.
On February 6, 2009, he re-signed with the Rangers to a minor league contract with an invitation to spring training.
On August 27, 2009, Jennings was designated for assignment, then given his outright release by the Rangers.