[4] After his redshirt year, he began to take on his role with the team, including working on returning kicks, with positive growth and success.

[17] Zenner entered training camp competing for the fourth running back on the Lions' depth chart with George Winn.

Zenner ultimately won the position, leading the NFL in rushing yards during the preseason, and went into the regular season as the backup behind Joique Bell, Theo Riddick, and Ameer Abdullah.

[20] On October 5, 2015, he carried the ball twice for nine yards and made his first career reception on a four-yard pass from Matt Stafford during a 10–13 loss to the Seattle Seahawks.

[21] The following week, Zenner earned his first career start and had a season-high ten carries for 30 rushing yards against the Arizona Cardinals.

[25] During the 2016 training camp, Zenner competed for the third backup running back position with rookie Dwayne Washington.

He was named the fourth running back on the depth chart to begin the season, behind Ameer Abdullah, Theo Riddick, and Dwayne Washington.

[27] The following week, he ran for 29 rushing yards on nine carries and scored his first career touchdown on a one-yard run in the third quarter of a 20–17 win against the Washington Redskins.

[44] After scoring 30 on the MCAT, he was admitted the University of South Dakota Sanford School of Medicine, but deferred entry to play in the NFL.

While on injured reserve in 2016, Zenner researched the effects of fructose on hypertension at Henry Ford Hospital in Detroit.

[45] Zenner has worked towards keeping children alive from the rare debilitating genetic disorder, Batten disease.