A native of Lake City, Florida, Jernigan attended Columbia High School, where he was an All-American defensive lineman and teammates with All-American offensive lineman Laremy Tunsil for his junior and senior year. As a junior, Jernigan registered 131 tackles, 27 tackles for loss, 12 sacks, three forced fumbles and three fumble recoveries and was a 3A All-State selection. In his senior season, he added 77 tackles, 32 tackles for loss, 14 sacks and one interception. Regarded nationally as one of the best defensive lineman in his class, he participated in the 2011 U.S. Army All-American Bowl and was a USA Today All-USA selection. He was also a member of the track & field team.

Still serving as Dawkins’ reserve in 2012, Jernigan was one of the leading tacklers on the team with 46. He also ranked fourth on FSU in tackles for loss with eight. In his final four games, Jernigan recorded 22 tackles, two tackles for loss and a sack. In the 2012 ACC Championship Game against Georgia Tech, he recorded the team’s only sack, was second on the squad with 1.5 TFL and was third on the team with nine tackles.

Jernigan led Florida State with 11 tackles for loss in 2013. Jernigan finished this season with 63 tackles, including 4.5 sacks, and was a second-team AP All-American while FSU won the national championship.

Jernigan announced on January 9, 2014 that he would forgo his senior season and enter the 2014 NFL Draft.

In 2016, Jernigan started 15 of 16 games, finishing second on the team with five sacks.

On April 4, 2017, the Ravens traded Jernigan to the Philadelphia Eagles along with the 99th overall pick in the 2017 NFL Draft in exchange for the Eagles third round pick (74th overall) in the 2017 NFL Draft. The Ravens selected Chris Wormley with the acquired selection.

After the 2017 season Jernigan had surgery on his back on May 3, 2018. He was placed on the reserve/non-football injury list to start the 2018 season. He was activated off the NFI list on November 20, 2018.

On March 4, 2019, the Eagles declined the option on Jernigan’s option, making him an unrestricted free agent. On April 25, 2019, the Eagles re-signed Jernigan to a one-year deal.

Jernigan signed with the Jacksonville Jaguars on August 17, 2020. On September 28, 2020, Jernigan was released by the Jaguars.

On September 30, 2020, Jernigan signed with the Denver Broncos. He was released on October 20, 2020.