Poppinga served as a defensive graduate assistant and intern on Mendenhall’s staff in 2009 and 2010 before taking over the outside linebacker responsibilities for the final eight games of the 2010 season.

After playing one year in the NFL, Poppinga returned to BYU for the 2009 season as the defensive intern and contributed to the team's 11-2 record and 44-20 Maaco Bowl Las Vegas victory over No. 16 Oregon State. After the team’s 1-4 start in 2010, Poppinga was asked as the defensive graduate assistant to assume the role as outside linebackers coach and helped the Cougars finish 6-2 over the remaining games, including a 52-24 win over UTEP at the New Mexico Bowl.

He helped BYU rank No. 13 in overall defense, No. 19 in rushing defense and No. 22 in scoring defense in 2011 before one of BYU's best-ever defenses took the field and finished with a No. 3 ranking in total defense in 2012 while ranking in the top 10 in six different defensive categories. 

In 2014 BYU ranked No. 20 in rushing defense and Poppinga helped a defensive unit finish the 2015 regular season ranked No. 6 nationaly in sacks per game and No. 15 in tackles for loss. Under Poppinga, BYU's special teams also ranked No. 7 in punting average and No. 19 in kick return coverage in 2014.

He helped outside linebackers Kyle Van Noy and Alani Fua advance to the NFL in recent years with Van Noy going to the Detriot Lions in the second round of the 2014 NFL Draft and Fua earning a spot with the Arizona Cardinals in 2015. 

Poppinga was an All-Mountain West Conference linebacker and Academic All-MWC honoree at BYU under Mendenhall while helping the Cougars achieve back-to-back conference championships in 2006 and 2007. He redshirted Mendenhall’s first season as head coach in 2005 after transferring from Utah State. Poppinga helped BYU achieve Las Vegas Bowl wins over Oregon and UCLA and led the team with 113 tackles his senior season. He played in the NFL in 2008, seeing time on three teams before going to Super Bowl XLIII with the Arizona Cardinals.

A native of Evanston, Wyo., Poppinga has completed bachelor’s (2007) and master’s (2010) degrees from BYU in exercise science. He and his wife Rebekah have two daughters.

Bronco Mendenhall on hiring Poppinga: “Kelly is a bright, aggressive and skilled football coach. He has demonstrated as a player and now as a coach that he demands excellence of himself and others. Kelly did an exceptional job this past season (2010) when asked to step into the role of coaching our outside linebackers and I’m excited to have him continue in that position.”