He was selected by the Colts in the fifth round of the 2005 NFL draft and later won Super Bowl XLI with the team.
Hagler attended Bishop McNamara High School in Kankakee.
As a senior, he had 3 sacks and 61 tackles on defense, and rushed for over 1,400 yards and 25 touchdowns on offense.
On November 4, 2009, it was reported that Hagler had suffered a ruptured biceps and would be on injured reserve for the rest of the season.
During a key week 15 game against the Jacksonville Jaguars, Hagler returned an onside kick from Josh Scobee 41 yards for a touchdown.