[3] A four-star recruit, Jennings committed to the University of Tennessee to play college football.

[4][5] Jennings played at Tennessee from 2015 to 2019 under head coaches Butch Jones and Jeremy Pruitt.

[8] Later on in the season, in a 28–27 loss to rival Florida, he showed off his dual-threat ability when he threw a 58-yard touchdown pass to quarterback Joshua Dobbs on a trick play.

[11] In the next game against rival Georgia, Jennings recorded the game-winning reception on a 43-yard Hail Mary from Dobbs to put the Vols 34–31 with no time remaining.

[13] In 2017, Jennings appeared in only one game due to a wrist injury but was dismissed from the team under interim head coach Brady Hoke later in the season.

He registered his first NFL touchdown in Week 2 against the Philadelphia Eagles, and by the end of November had become an integral part of the offense.

[31] His production continued in the playoffs, with Jennings recording at least one reception in each round before the 49ers exited in the National Football Conference (NFC) Championship.

[37] He also threw a 21-yard touchdown pass to Christian McCaffrey in a 25–22 overtime loss to the Kansas City Chiefs.

However, he was prevented from achieving his first-ever 1000-yard season after getting ejected midway through a Week 18 game against the Arizona Cardinals.