He received his Juris Doctor in 1979 from the Pace University School of Law.

He spent most of the period from 1986 to 2001 serving as an assistant United States attorney in the Middle District of Pennsylvania, except from 1993 to 1997, during which he was a partner at the law firm of Hourigan, Kluger, Spohrer & Quinn PC.

[3][2] Additionally, he was chief magistrate judge of the district court from 2001 to 2012.

[1][4] On May 17, 2012, President Barack Obama nominated Mannion to serve as a United States District Judge for the United States District Court for the Middle District of Pennsylvania, to the seat vacated by Judge Richard Caputo, who took senior status in 2009.

The Senate Judiciary Committee held a hearing on his nomination on June 27, 2012, and reported it to the floor on July 19, 2012.