Jennings, who only joined up with the squad as injury cover this week, looked an old hand in an assured 112 that put England on the front foot on day one before the loss of late wickets left them on 288-5.
Just days earlier, the South Africa-born 24-year-old had been celebrating a Test call-up by scoring his first hundred for England Lions against the United Arab Emirates in Dubai.
Now the Durham left-hander is the first England opener to register a ton on debut since captain Alastair Cook, in 2006, and he concedes that his meteoric rise is becoming difficult to absorb.
“It has been an incredible six to eight months and probably an even more special 72 hours, flying in from Dubai, meeting the guys and getting a hundred today.
“If someone had said to me at the end of September you would score as many runs as you had and have the opportunity with the Lions, I would have snapped their hand off.
“If someone said to me there would be a Test hundred waiting a couple of weeks later I would have jumped all over it. It’s the best Christmas present I could get.”
Jennings survived a dropped catch at gully when still scoreless and later a close lbw call before cutting loose with an array of reverse-sweeps and cover drives allied to more prosaic hitting.
He put on 99 with Cook and 94 with Moeen Ali (50) as England made the most of winning the toss on a spinners’ pitch in a match they must win to retain any hope of drawing the five-match series.
Cook, for whom Jennings was an 11th different opening partner in four years, made 46 before departing in unusually cavalier fashion, stumped as he advanced down the wicket to a Ravindra Jadeja delivery.
Root made 21 before becoming the first of Ravichandran Ashwin’s four victims (4-75), caught at slip by Virat Kohli, signalling Moeen’s arrival for another extended stand with Jennings.
But Moeen’s attempted slow-sweep off Ashwin was caught by Karun Nair and England quickly slipped from a commanding 230-2 to 249-5.
Jennings – watched from the stands by Haseeb Hameed, the man he replaced after the Lancashire teenager suffered a hand injury in the third Test – went two balls after Moeen, edging Ashwin to Cheteshwar Pujara at gully.
Jonny Bairstow was caught on 14 by Umar Yadav at deep backward square leg after a miscued sweep, before Ben Stokes (25) and Jos Buttler (18) saw England to stumps.