Brandon King played at the linebacker position at Thompson High School in Alabama and for the Highland Scotties of the Kansas Jayhawk Community College Conference.

[1] In July 2012, King verbally committed to transfer to the University of South Alabama and to play for the Jaguars football team.

[3] At Auburn, King played behind Robenson Therezie at the "star" safety position, seeing action in every game of his first season for the Tigers, albeit mostly on special teams.

On March 3, 2015, King participated at Auburn's pro day, along with Cameron Artis-Payne, Angelo Blackson, Quan Bray, Sammie Coates, Reese Dismukes, Corey Grant, Nick Marshall, C. J. Uzomah, Jermaine Whitehead, Gabe Wright, Trovon Reed, and nine other teammates.

On September 5, 2015, King was released by the Patriots as part of their final roster cuts and was signed to the practice squad the next day.

He forced the Bills' punt returner Leodis McKelvin to fumble and it was recovered by Patriots' linebacker Jonathan Freeny.

[12] King finished his rookie season with 12 combined tackles (ten solo) in 13 games and played solely on special teams.

The contract terms were for one year and $525,000[15][8] King competed with Cedric Thompson throughout training camp for a job as a backup strong safety.

On October 29, 2017, King forced his first career safety while defending a punt return when he tackled Travis Benjamin in the endzone during the Patriots' 21–13 victory over the Los Angeles Chargers.

[22] King helped the Patriots reach Super Bowl LIII, which they ended up winning against the Los Angeles Rams by a score of 13–3 on February 3, 2019.