Hours after being handed a chance to get closer to the final spot in the Eastern Conference when Atlanta blew a fourth-quarter lead in Toronto, the Knicks responded with an inept final period of their own after they led by 17 points in the first half.
They couldn't stop Jack and couldn't get a basket from Carmelo Anthony, who missed all five shots in the final period and finished with 32 points.
Jack scored 23 in the second half and finished with 10 assists for the Cavaliers, who snapped a four-game losing streak.
Dion Waiters scored 22 points and Luol Deng had 13 in his return from a three-game absence with a sprained left ankle for the Cavs, who didn't miss All-Star Kyrie Irvingbecause Jack torched the Knicks for 13 points in the fourth quarter.
New York was missing ailing Iman Shumpert, their best perimeter defender who surely would've gotten a chance to defend Jack since nobody else could.
The Knicks, who suddenly were back in the playoff race after looking out of it after a seven-game skid dropped them to 21-40 after a loss in Detroit on March 3, will have to bounce back on the road. They leave for a five-game Western Conference trip that begins Tuesday against the Lakers.
The Hawks had already lost at Toronto by the time the Knicks showed up for work Sunday. Coach Mike Woodson said he was glad but didn't appear overjoyed, realizing the Knicks still have plenty to do to get to the 35 to 40 wins he think will be necessary to earn a playoff spot.
"It's good that they lose, but again, it's about what we do at this point," Woodson said before the game.
Even getting to 35 wins will be difficult considering they still have games against Miami and Chicago, plus two apiece against Brooklyn and Toronto after their trip.
New York made 15 of 21 shots (71 percent) in the first quarter, getting 14 points from Anthony and taking a 34-27 lead when J.R. Smith made a 28-footer with 0.2 seconds left. The Knicks pushed the lead to 17 in the second when Pablo Prigioni made a 3-pointer, then stole the ball and threw an alley-oop pass that Smith slammed down to finish off a fast break and make it 51-34.
But down 61-46 at halftime, Cleveland scored the first nine of the second half to get within six, and the Knicks never really got the momentum back.
The Cavs used an 11-4 spurt to tie it at 90 on Jack's jumper with 7:29 remaining, then Deng's 3-pointer gave them a 93-92 edge with 5:46 to play.
Anthony's free throws gave the Knicks their last lead, but Jack knocked down a jumper, Waiters made a 3-pointer and Anderson Varejao had an easy dunk after a pick-and-roll to make it 100-94 with 2:37 remaining.
New York cut it to two, but Jack answered with another jumper, finishing two points shy of his career high.