[3] Johnson was a walk-on for the Tar Heels at the University of North Carolina at Chapel Hill in 2004, where he competed as a reserve quarterback behind T. J. Yates before graduating in 2008 with degrees in mathematics and computer science.
[3][5] After college graduation, Johnson spent a year not in football, working instead as a software developer in Durham, North Carolina for the company eTeleNext.
[6][7] Inspired to get into coaching by his offensive coordinator at UNC, John Shoop, Johnson was hired as a graduate assistant for the Boston College Eagles in 2009.
On January 30, 2024, Johnson announced he would be returning to the Lions despite being considered a prime head coaching candidate for both the Washington Commanders and Seattle Seahawks.
After the regular season, Johnson interviewed with the Chicago Bears, Jacksonville Jaguars, Las Vegas Raiders, and New England Patriots for their vacant head coach positions.
[20] Johnson cited his offensive scheme as being influenced by Kevin Rogers, Darrell Bevell, Adam Gase, Clyde Christensen, and Mike Martz.