Kyogo Furuhashi gave Celtic the lead in the 48th minute but Alex Iacovitti levelled with Ross County’s first effort on target 10 minutes later.
Malky Mackay’s side looked like they might frustrate their visitors but Jenz ran in to meet Jota’s inswinging cross and head into the roof of the net from six yards in the 84th minute.
Substitute Liel Abada wrapped up victory in stoppage time when he curled home from 18 yards.
On-loan Lorient defender Jenz had only been drafted in after Stephen Welsh dropped out through illness on Friday.
The 23-year-old German picked up a booking in a shaky moment in defence but looked assured on the ball and made himself an instant hero with the crucial moment of the game on a ground where Celtic needed an Anthony Ralston winner in the seventh minute of stoppage time last season.
Andrew Smith was in Dingwall and rates the Celtic players’ performances out of ten ...
5. Greg Taylor
Busy throughout, the ease with which he steps inside to play the inverted full-back role helped his attackers pull County players out of position. 7 Photo: Rob Casey - SNS Group
6. Callum McGregor
The crispness and immaculate ball circulation normally a given from the Celtic captain wasn’t always in evidence, but the willing was certainly there. 6 Photo: Craig Foy - SNS Group
7. David Turnbull
Starting in place of the injured Reo Hatate, he appeared the midfielder most likely to make things happen in curious first half, before giving way to Giorgos Giakoumakis in the 73rd minute as Celtic chased a winning goal. 7 Photo: Craig Williamson - SNS Group
8. Matt O'Riley
His growing reputation may be preceding him as close attention from opponents meant he couldn’t always get on the ball and play telling passes as his game is built around. 6 Photo: Craig Williamson - SNS Group