Lyth, who hit 125 against Northants and 136 against Lancashire on successive days in June, made 88 as Yorkshire batted first in the day/night fixture at the St Lawrence ground.
Yorkshire’s total also featured 45 from England vice-captain Joe Root, making his first Royal London appearance of the season.
Jonny Bairstow scored nine on what was also his first appearance in this year’s tournament, both England men falling to Charlie Hartley, a 22-year-old pace bowler playing only his fourth List A match.
Yorkshire, 129-2 at halfway, did not manage the big finish they had threatened as Kent fought back impressively.
There was 37 for England left-hander Gary Ballance, with Matt Coles returning Kent’s best figures of 3-39.
In perfect conditions in the south-east, where a capacity crowd basked in glorious sunshine, Yorkshire were put into bat by Sam Northeast.
Alex Lees, the Yorkshire captain, said in an interview conducted at the toss that he would have batted in any case, but his own contribution was cut short in the fifth over when he pulled Mitch Claydon, the former Yorkshire pace bowler, to Alex Blake at deep mid-wicket.
The shot was on for Lees, but he thumped the ball straight at the man to depart for seven with the score on 27.
Lyth, his opening partner, looked in great touch, which he showed by flicking Claydon for four over mid-off and then square-driving his next delivery to the cover boundary.
A six followed when Claydon was contemptuously dispatched over long-off before Coles, the other opening bowler, was cover-driven and pulled to the boundary by the Yorkshire left-hander.
Lyth advanced to his half-century from 58 balls, reaching the milestone with a rare false stroke when he sliced spinner James Tredwell over the vacant slip area.
Lyth and Root added 90 for the second wicket in 19 overs before Root perished on the mid-wicket boundary, well caught by Blake off Hartley.
Hartley struck again when Bairstow chipped to mid-on, where Daniel Bell-Drummond took a comfortable catch.
Yorkshire fell to 167-4 in the 34th when Lyth flirted at one outside off stump off the returning Coles and was caught behind.
The visitors lost two wickets with the score on 191 in the 39th over, bowled by Will Gidman, when Tim Bresnan chipped to Northeast at cover for 10 and David Willey was strangled down the leg-side for a golden duck.
Hartley might have had a third wicket when Liam Plunkett, on one, fired the ball straight back at him, but the bowler was unable to hold on.
Plunkett went on to 16 before skying Coles to Northeast at cover, and Yorkshire fell to 237-8 in the 48th when Ballance was run-out for 37.
Adil Rashid played Claydon out into the leg-side, and Ballance was run-out in a mix-up over whether the pair should attempt a third run.
Coles bowled Azeem Rafiq for a single before Yorkshire took 14 off the last over, bowled by Claydon, Rashid finishing unbeaten on 24 from 16 balls.