Nielsen began his coaching career as a volunteer assistant at his alma mater USC in 2002. He also spent time as an administrative assistant at Idaho under former USC assistant Nick Holt. He was hired away as a graduate assistant at Ole Miss under his position coach at USC, Ed Orgeron, and was promoted to defensive line coach before the season began.[3] He also had stints at Central Connecticut State and UT Martin before being hired at Northern Illinois in 2011 as a co-defensive coordinator.[4] He followed Huskies head coach Dave Doeren to NC State in 2013 to serve as his defensive line coach and recruiting coordinator.[5] He added the title of run game coordinator in 2014.
New Orleans Saints
Nielsen was hired as the defensive line coach for the New Orleans Saints on February 9, 2017.[6]
Nielsen was offered the defensive coordinator position at LSU in 2021, but elected to remain with the Saints where he was promoted with the added title of assistant head coach in 2021.[7][8]
Nielsen was promoted to co-defensive coordinator for the New Orleans Saints in 2022.
Atlanta Falcons
On January 27, 2023, Nielsen was hired as defensive coordinator for the Atlanta Falcons.
In Nielsen’s first year with the Falcons, the overall defensive rankings improved from 23rd to 18th in points allowed per game and from 27th to 11th in yards allowed per game.[9] The Falcons also doubled their sack total (42) from the previous season (21).[10][11]
Jacksonville Jaguars
On January 22, 2024, Nielsen was hired as defensive coordinator for the Jacksonville Jaguars.[12]