[2] Lacking the grades to enroll at a Division I school, McCullers played his freshman and sophomore seasons at Georgia Military College.

In 2011, during his sophomore year, he recorded 37 tackles, including nine tackles-for-loss and two sacks, as part of a defense that held opponents to just 261 yards per game.

[2] After Tennessee head coach Derek Dooley was fired late in the 2012 season, there was speculation that McCullers would forgo his final year of eligibility and enter the NFL Draft.

He recorded six tackles in Tennessee's upset of South Carolina, including a sack that knocked quarterback Connor Shaw out of the game.

He was the subject of a USA Today feature in September 2012,[3] and a photograph of McCullers towering over a Tennessee trainer was widely circulated on the web.

[14] McCullers wore #74 during the preseason, then changed to #92 for the regular season, but then swapped his number to #62 when James Harrison, who had previously worn #92 with Pittsburgh, re-signed with the Steelers.

After McLendon signed with the New York Jets in the 2016 offseason, McCullers switched his jersey from #62 to #93 and competed for the starting defensive tackle position with rookie Javon Hargrave.

On October 16, 2016, McCullers recorded his first career blocked field goal against the Miami Dolphins, deflecting Andrew Franks' 24-yard attempt.

On January 1, 2017, McCullers registered his first full career sack in the Steelers' regular season finale against the Cleveland Browns, tackling quarterback Robert Griffin III for a loss of 8 yards on a 3rd and 5 in the fourth quarter.

The sack proved costly for the Browns, as Cody Parkey missed a 49-yard field goal on the following play in a game the Steelers would eventually win 27–24 in overtime.

[25][26] On September 24, 2020, McCullers was signed off the Steelers practice squad by the Chicago Bears to replace John Jenkins, who was placed on the injured reserve list due to a ligament tear in his thumb.