High School.... Jennings attended Poteet High School in Mesquite, Texas, and was a standout in both football and baseball. In football, he won All-Area honors at both, kicker and punter. In baseball, as a senior, he was named the District MVP and posted a .410 batting average, hit seven home runs, and pitched his way to a 10-3 record and a 0.92 ERA with 132 strikeouts.

College.... Jennings attended Baylor University where he played baseball from 1997-1999. Following his junior season at Baylor, Jennings was named by 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 his second consecutive Big 12 Player of the Year, consensus All-American honors, the Outstanding Player on the 1999 Big 12 All Tournament Team, and a spot on the 1999 All-Big 12 Academic First Team. His college career was capped off by winning the Golden Spikes award from USA Baseball, considered the Heisman Trophy of college baseball.

Professional career....As a member of the Colorado Rockies, Jason posted his most impressive numbers in the 2002 season when he won the MLB Rookie of the Year Award in the National League, beating out Brad Wilkerson and Austin Kearns. In his major league debut, Jennings hit a home run and tossed a complete game shutout, becoming the first major leaguer to accomplish that feat. In 2006, Jennings ended 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.

Jennings and his wife, Kelly, have three children, son Braden, son Keathan, and daughter Bailee. His father, Jim, played baseball for the University of Texas and professionally in the Texas Rangers system. His late grandfather, James Jennings, was the long-time public address announcer for the Dallas Cowboys and Mesquite Championship Rodeo.