During the indoor season, he competed in the 55-meter dash and recorded a personal-best time of 7.01 seconds at the Oshkosh HS Invite.

[8] Throughout training camp, Klein competed against Chase Blackburn, Jordan Senn, Jason Williams, and Doug Hogue for a role as the primary backup outside linebacker.

[9][10] Head coach Ron Rivera named Klein the backup strongside linebacker, behind Thomas Davis, to begin the regular season.

[11] He made his professional regular season debut during the Carolina Panthers' season-opening 12–7 loss against the Seattle Seahawks.

On November 10, 2013, Klein earned his first career start after Chase Blackburn was ruled inactive due to a foot injury he sustained the previous week.

Klein collected a season-high five solo tackles and made his first career sack during their 10–9 victory at the San Francisco 49ers in Week 10.

[18] Klein entered training camp slated as the starting weakside linebacker, but saw competition from rookie first round pick Shaq Thompson.

[19] Head coach Ron Rivera named Klein the backup weakside linebacker, behind Shaq Thompson, to begin the regular season.

[20] On September 20, 2015, Klein started at middle linebacker after Luke Kuechly was inactive due to a concussion he sustained the previous week.

[21] He recorded four combined tackles, one pass deflection, and made his first career interception as the Panthers defeated the Houston Texans 24–17 in Week 2.

Klein intercepted a pass by Texans' quarterback Ryan Mallett, that was intended for tight end Garrett Graham, and returned it eight yards during the fourth quarter.

[23] Klein finished the season with 55 combined tackles (39 solo), one pass deflection, one sack, and one interception in 15 games and six starts.

[30] In Week 10 against the Arizona Cardinals, Klein recorded a team high 10 tackles and sacked Kyler Murray once during the 32–30 loss.

[36] On October 6, 2022, the Baltimore Ravens signed Klein to their active roster off the Giants practice squad.

Before he was called up, he had bought an RV and was planning a road trip to Key West, Florida, for the weekend.

[50] Klein and his wife, Taylor, have a son, Cannon James, whom they adopted in 2019, and a daughter, Bowie Rose, also born in 2019.