John grew up in Magee, MS and went to Magee High School, where he won a 3A State Championship. He attended the University of Alabama to play football. While at Alabama, Mangum was named All SEC multiple seasons, set the single season (24) and all time (47) record for pass break-ups, and set the school record for career interceptions (16).

In 1990, he was drafted in the 6th round of the NFL draft by the Chicago Bears. He spent 9 seasons with the Bears. Over his NFL career, Mangum played in 105 games, had 5 interceptions, 1 kickoff return, and 4.5 sacks.

In addition to being inducted into the Mississippi Sports Hall of Fame, John has also been named an Alabama Power Legend and the Birmingham Post Herald Alabama Team of the Decade.