John Fencik is a former professional free safety. He played college football at Yale University, where he received his bachelor's degree in 1976.

Fencik played 12 seasons with the Chicago Bears and as of 2021, is their all-time leader in interceptions and total tackles. John Madden once said in a broadcast that "Gary Fencik played football at Yale; that is like saying clean dirt". Fencik was originally drafted by the Miami Dolphins in the tenth round of the 1976 NFL Draft, with the 281st overall selection. After rupturing his left lung in a preseason game against the New Orleans Saints, he was released in September and picked up by the Bears. In Chicago, he was the team's defensive captain through the 1980s including the 1985 Super Bowl championship season. He made two Pro Bowl appearances (1980-1981). He was also awarded a gold record and a platinum video award for the 1985 Super Bowl Shuffle. Fencik and Doug Plank were dubbed "The Hit Men", a fact referenced by Fencik in The Super Bowl Shuffle. Fencik finished his career with 38 interceptions, which he returned for 488 yards and a touchdown. He also recorded 2 sacks and recovered 14 fumbles, returning them for 65 yards.

Following his football career, Fencik has worked in the finance industry. Before joining Adams Street Partners in 1995, Fencik worked with Wells Fargo and UBS. He has also worked as a sports commentator.