Allen was selected by the Kansas City Chiefs in the second round of the 2012 NFL draft.

After graduating from Chicago's King High School, Allen attended the University of Illinois at Urbana–Champaign from 2008 to 2011.

[3][4] In his rookie season, he appeared in all 16 games, and became a starter at left guard by Week 4 against the San Diego Chargers.

For his first two seasons with the Chiefs, he played with Jon Asamoah who was a teammate of Allen's at Illinois.

[5][6] On May 11, 2018, Allen was placed on the reserve/physically unable to perform list after dealing with concussions and ankle injuries.