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 ...
1. Joe Hart
Seemed to be a little unsure of himself at the corner from which Celtic conceded but maybe his concentration had wavered with so little involvement till that point. 6
Photo: Alan Harvey - SNS Group
2. Josip Juranovic
The tight pitch at Dingwall wasn’t conducive to the sort of over-lapping runs that are a key feature of the Croatian right-back’s game but he certainly worked hard enough. 6
Photo: Craig Williamson - SNS Group
3. Moritz Jenz
A day to remember for the German centre-back thanks to his crashing 84th minute headed goal. It capped a thoroughly decent debut, the player purring through the opening half before a few splutters as he earned a booking in coming under a little more pressure in the second 45. 7
Photo: Craig Foy - SNS Group
4. Cameron Carter-Vickers
So much were County clamping down at the back, he took matters into his own hands with a mazy run into the box that almost led to a goal. Little to do defensively. 6
Photo: Rob Casey - SNS Group