Nielsen got his coaching start at the college level. He spent two seasons at Northern Illinois and four seasons as the defensive line coach, recruiting coordinator and run game coordinator at North Carolina State. Nielsen left the college ranks to join the Saints coaching staff as their defensive line coach, a position he has held since 2017. It was there that Nielsen met Falcons general manager Terry Fontenot, who spent 18 seasons in the Saints front office.

The Saints defensive front has been one of the league's best for the last decade. Since Nielsen joined the Saints staff in 2017, New Orleans has recorded 281 sacks – that's the second most in the NFL over that span – and finished in the top 10 in sacks in five of the last six seasons.

Also, during Nielsen's time in New Orleans from 2017 to 2020, the Saints put together 55 regular season and postseason games without allowing a 100-yard rusher.

Ryan Nielsen was hired as the Atlanta Falcons defensive coordinator on Jan. 27, 2023.

Nielsen comes to Atlanta having most recently served as the co-defensive coordinator and defensive line coach for the New Orleans Saints. Since Nielsen joined the Saints staff in 2017, New Orleans has recorded 281.0 sacks – second-most in the NFL over that span – and finished in the top 10 in sacks in five of six seasons. The Saints also did not allow a 100-yard rusher in an NFL-record 55 regular-season and postseason games between 2017 and 2020 and have finished in the top four in run defense in four of the past five seasons.

Since Nielsen joined the staff in 2017, New Orleans has ranked third in opponent rushing yards per game and fifth in opponent yards per rush. Since 2017, the Saints are the only team to rank in the top three in both opponent rushing yards per game and cumulative sacks over the six-season period. Additionally, New Orleans has ranked ninth or better in scoring defense in each of the last three seasons.

In his six seasons working with New Orleans' defensive line, Cameron Jordan was named to six Pro Bowls, earned second-team All-Pro honors twice and became the first Saints defensive lineman to earn first-team All-Pro honors in franchise history. Nielsen also worked with two-time Pro Bowl defensive end Trey Hendrickson, who had a career season in 2020 with New Orleans. Hendrickson finished with 25 total tackles, 13.5 sacks and 25 quarterback hits under Nielsen's tutelage in 2020.

Prior to joining the Saints, Nielsen served as the defensive line/recruiting coordinator/run game coordinator at North Carolina State for four seasons (2013-16). In 2016, the Wolfpack ranked eighth in the nation in rushing defense and lead the ACC, limiting opponents to 108.6 rushing yards per game — the school's best mark since 2004. Junior defensive end Bradley Chubb recorded 22 tackles for loss, fourth-most in the country, and 11.0 sacks, fourth-most in the ACC. Chubb earned second-team All-ACC honors that season.

A Broyles Award candidate in 2015 and 2016, given annually to college football's top assistant coach, Nielsen earned this recognition tutoring a Wolfpack defensive line that was part of a defense that ranked 29th in the nation in opponent net yards per game. Defensive end Mike Rose was a second-team All-ACC selection after he finished second in the ACC in sacks and ninth in tackles for loss. In the 2018 NFL Draft, four linemen who Nielsen coached in Raleigh – Chubb, B.J. Hill, Justin Jones and Kentavius Street – were all selected.

Nielsen coached at Northern Illinois from 2011-12, including a 2012 season where he was the co-defensive coordinator/defensive line/recruiting coordinator and helped the Huskies to the GoDaddy.com Bowl. Two defensive linemen were first-team All-MAC selections in 2012 – Sean Progar and Alan Baxter – while Nabal Jefferson was a National Scholar-Athlete and finalist for the 2012 William V. Campbell Trophy. In 2011, Nielsen coached Progar and Ron Newcomb to All-Mid American Conference honors.

Serving as the defensive line/special teams coach at Tennessee-Martin in 2010, the Skyhawks led the Ohio Valley Conference in total defense. From 2008-09, Nielsen was the defensive coordinator and coached the defensive line and linebackers at Central Connecticut State. In 2009, Central Connecticut won the Northeastern Conference title. Nielsen served as the defensive line coach at the University of Mississippi from 2005-07 under Ed Orgeron, following one season at the University of Idaho (2004), where he coached two freshman All-Americans. Mississippi defensive linemen led the Southeastern Conference in sacks twice during his three-year stint.

Nielsen got his start in coaching as a volunteer assistant at his alma mater, the University of Southern California in 2002. Nielsen, a Simi Valley, Calif. native who started 30 games at defensive tackle at USC from 1998-2001, finished his college career with 107 career tackles and was voted the team's Defensive Lineman of the Year in 1999. He graduated with a bachelor's degree in public policy in 2002 and went to training camp with the Philadelphia Eagles. He played for the Arena Football League's Los Angeles Avengers in 2004.