Florida chose Beckett with the second overall pick in the 1999 amateur draft. After compiling a combined 14-1 record with a 1.54 ERA for two minor league teams in 2001, he earned his first call-up to the parent club. In four September starts, Beckett racked up a 2-2 record with a 1.50 ERA and a 1.042 WHIP, striking out 24 in 24 innings.
2002 would see Beckett join the opening day club as the number five starter. Slowed by injury, Beckett posted a 6-7 record with a 4.10 ERA, striking out 9.4 batters per nine innings pitched.
Beckett would post better numbers in 2003, finishing with a 9-8 record and a 3.04 ERA, again striking out more batters than innings pitched. On the way to the Marlins second World Series Championship, Beckett contributed to the cause, going 2-2 with a 2.11 ERA, striking out almost 10 batters per nine innings in the postseason.
His best season with Florida was 2005, in which he posted a 15-8 record and a 3.38 ERA with a slightly above average WHIP of 1.181.
After the 2005 season, Beckett was traded with Mike Lowell and Guillermo Mota to the Boston Red Sox for Jesus Delgado, Harvey Garcia, Hanley Ramirez and Anibal Sanchez.
In each of Beckett's four full seasons with the Marlins, he suffered an injury in the month of June. He would have been considerably higher on this list had he been able to avoid the injury bug. He never made 30 starts as a member of the Marlins rotation. By contrast, in four of his six seasons in Boston, Beckett has made at least 30 starts on four occasions, winning three All-Star selections and pitching for 20 wins in 2007, finishing second in the AL Cy Young award voting.
He collected a total record of 41-34, pitching in a total of 106 games for the Marlins, allowing 7.8 hits per nine innings pitched. He is currently signed with the Boston Red Sox through the 2014 season.