He played college football for San Diego State University.
Allen was signed as an undrafted free agent by the Jacksonville Jaguars on April 26, 2009.
On September 5, 2009, he became one of two undrafted rookies, the other being Julius Williams, to make the Jaguars opening day roster.
Allen was named starting weakside linebacker on October 11, 2009 against the Seattle Seahawks.
[1] Allen suffered a stroke on December 15, 2013 during a game against the Buffalo Bills.