Weaver brings a large arsenal out to the mound. The lanky righthander can throw a fastball in the low 90's, a hard and tight slider, and a sweeping breaking ball. Occasionally he will drop down to a sidearm delivery to try and catch hitters off guard. Weaver is known for allowing bad calls to upset him and throw him off his game. Critics say Weaver's tendency to think too much gets in the way of his natural talent.

Weaver was drafted in the first round (14th overall) in 1998 by the Detroit Tigers. He became a major component of their pitching staff in 1999 and improved his stats every season until they traded him just before the trade deadline in 2002. The trade involved three teams (the Tigers, A's and Yankees) and Jeff landed with the Yankees.

Jeff's time with the Yankees was very turbulent. They bounced him in and out of the starting rotation, even sitting him in the bullpen for weeks at a time. Then, during game 4 of the 2003 World Series, he was brought in and allowed a 12th inning walk off home run to Alex Gonzalez. This performance pretty much sealed Weaver's fate, and in the offseason, he was traded to the Los Angeles Dodgers.

In 2005, the Dodgers built their rotation around the durable Weaver (who has never been placed on the disabled list). Jeff went 14-11, with a 4.22 ERA, a 1.17 WHIP, 3 complete games and a career-high 157 strikeouts in 224 innings. These numbers were good enough to rank him in the NL as 8th in IP's, 9th in complete games and 9th in WHIP. Following the 2005 season, Jeff filed for free agency. The Dodgers offered Jeff salary arbitration but the two side were unable to come to an agreement.

On February 15, 2006, Weaver signed a 1-year deal with the Angels for $8.5-million.

In 2004, Jeff's younger brother, Jered Weaver, was drafted 12th overall in the first round by the Los Angeles Angels of Anaheim.