He played in Major League Baseball (MLB) for the Kansas City Royals, Tampa Bay Devil Rays/Rays, Los Angeles Dodgers, and Toronto Blue Jays.

Howell attended Jesuit High School in Sacramento, California where he was a four-year varsity letterwinner.

[2] He set a sectional playoff record with 47 strikeouts in 22 innings in 2000 as a junior and played in various international tournaments, including winning silver medals at the 2001 Junior Pan American Games, the 2000 International Baseball Association Foundation Games and the 1998 and 1999 AAU National Championships.

[11] He made his professional debut with the Idaho Falls Chukars of the Pioneer League in 2004, appearing in six games (four as a starter) with a 3–1 record and 2.77 ERA.

[11] He began 2006 back with Omaha, where he was 3–2 with a 4.75 ERA in eight starts,[12] missing the month of May as a result of shoulder stiffness.

[15] On June 20, 2006 Howell was traded to the Tampa Bay Devil Rays for infielder Fernando Cortez and outfielder Joey Gathright.

[11] It was the first trade by new Royals general manager Dayton Moore who said the deal was made to improve the outfield defense.

Devil Rays vice-president Andrew Friedman said that he envisioned Howell pitching in their starting rotation at some point.

[12] He made his Devil Rays debut on August 1, 2006 against the Detroit Tigers, allowing six runs in only 32⁄3 innings.

[11] In 2010, Howell suffered from what was reported as "weakness" in his left shoulder during spring training, causing him to begin the season on the disabled list.

[23] A few days later he underwent surgery on the shoulder to repair a torn labrum, causing him to miss the entire 2010 season.

[24] Howell was expected to miss the start of the following season as well[25] and was non-tendered by the Rays on December 10, 2010, to avoid arbitration.

He also throws a knuckle-curve that has been described by scouts as "heavy" because of its extremely sharp downward break, and some even classify it as a slider.

In 619.2 innings pitched covering 547 games, Howell committed only one error in 146 total chances for a .993 fielding percentage.