Jennings was considered fortunate to retain his place after a torrid summer but mined all his reserves of resilience to make 146 not out in 280 balls, anchoring an innings of 322-6 declared for more than six hours and leaving the home side chasing a world record 462.
They successfully negotiated seven overs for 15 without loss by stumps but the examination has two full days to run, regional weather systems allowing.
Jennings’ second Test ton was neither pretty nor dynamic - and offered no clues about how he might reverse his struggles on seaming decks at home - but it was a study in concentration, the kind of effort England might have feared had headed for retirement with Alastair Cook.
The left-hander rarely came out of his shell en route to three figures, scoring 59 of his first hundred in singles and relying on a steady diet of reverse sweeps and watchful pushes.
He offered only one chance on 58, when he would have been lbw on review had Sri Lanka not been dissuaded by the sound of bat on ball. Replays suggested that came after contact with the bat and showed the ball knocking over middle stump, but by then the moment had passed.
Jennings had seven partners in total, Ben Stokes the most productive with a bustling 62, while Ben Foakes’ late cameo saw him clearing the ropes three times - showing off yet another string to his bow on his remarkable debut. If Jonny Bairstow does return to fitness in Kandy next week, Foakes will surely not be the one to make way.
England started the day 177 in front and fought out a roughly even morning session, adding 73 for three wickets.
Jennings gritted his teeth and held his end up, happy to nudge his side towards a winning position rather than attempt anything outside of his comfort zone.
It brought him a modest 34 runs in the two hours to lunch, with a solitary boundary, but also saw him outlast three partners.
Rory Burns was the first, run out by Dimuth Karunaratne chasing an eager single after getting tied down by spin.
Following a soft dismissal for nine in his maiden knock he will perhaps see his maiden Test as a missed opportunity, though there should be plenty more to come.
Moeen Ali avoided a pair but did not linger, desperately hacking Dilruwan Perera to mid-on for three attempting to up the ante. His latest ill-starred experiment at number three has been a bust and someone else must be forced to shoulder the burden next time around.
Joe Root was the key wicket and retiring hero Rangana Herath claimed it for the second time in the match, gripping the surface and taking the edge as the captain pushed forward.
Jennings had probably earned a slice of luck and it came when Sri Lanka declined to refer Dhananjaya de Silva’s sound lbw appeal just before the break.
England took full control there after, adding 101 for one between lunch and tea and 110 for two in the evening, before declaring.
Stokes was first to put his foot down, strolling through the gears and lofting three sixes with punishing force.
He briefly threatened to overtake his partner, who was sticking steadfastly to his chosen method. Perera ensured that would not be the case, pitching one outside leg and clipping the top of off as Stokes tried and failed to cover the spin.
Jennings played out five balls on 98 before tea but the wait did not knock him off his groove and he safely strolled through for two more runs after play resumed.
The remainder of the innings was a procession against a weary fielding unit, Jennings adding his next 46 runs at a relative canter alongside Jos Buttler (35) and Foakes (37).