It was the complete performance from Ugo Ehiogu's team who sparkled for a 20-minute spell in the first half to lead 2-0 and then showed the other side of the game, defending stoutly against Sunderland's aerial threat before securing victory late on.
Harrison was at the heart of it with an all-round striker's treble - tapping home for 1-0, planting home a header for 2-0 and then a clinical drive from a tight angle to secure the points late on.
The victory was no less than the team deserved and after a tricky run of late, they showed their true colours on the main stage.
Sunderland had the early chances in a first half that ebbed and flowed.
Mikael Mandron's overhead kick flashed wide on five minutes and the same player was sent clear somewhat fortuitously from Martin Smith's block tackle minutes later but Tom Glover had the measure of his rising shot.
We were growing into the game when we opened the scoring on 13 minutes. The goal owed much to the alertness of Anthony Georgiou, who pounced when Joshua Robson's header home fell short, beat goalkeeper Maksymilian Stryjek darting out and then, with defenders back in place, he had the presence of mind to square for Harrison to slot home.
Will Miller had to clear off the line from Mandron but almost immediately played a key role at the other end as we forged 2-0 ahead. Miller read Stryjek's wayward throw and measured a cross that Harrison rose to head into the far corner.
The team now played with confidence and almost added a third when a lovely move ended with Harrison's flick into Kyle Walker-Peters, whose clean strike from 16 yards was parried out by Stryjek. Miller was racing in for any potential rebound but couldn't turn the ball back on target.
Unfortunately, a potential 3-0 lead quickly turned into 2-1 as Glover's pass was intercepted by Greenwood, who squared for Honeyman to tap home.
That changed the momentum of the half and Glover had to quickly atone for his error with a superb, point-blank save from Mandron's header.
The final chance fell to the visitors - always a threat at set-pieces - when Greenwood's free-kick was headed back by George Brady only for Carl Lawson to rattle the crossbar from six yards.
Sunderland started the second half with a spring in their step but we defended well and the only chance fell to Mandron, who angled Lawson's cross wide from six yards.
Winks and Miller tried their luck from distance - both a yard off target - but then a golden chance for Harrison as quick feet from Miller opened it up for the striker who connected well with his left foot, Stryjek turned away. Play switched ends and Glover got down well to deny Honeyman.
Miller prompted more good play 30 yards out on 68 minutes, opening it up for Harrison, who lashed a yard over.
We continued to defend well and Sunderland's next chance didn't arrive until the 79th minute when Greenwood sliced a half-volley into Park Lane.
Under-18 regular George Marsh entered the fray for his Under-21 debut for Emmanuel Sonupe on 82 minutes.
Inevitably, Sunderland ramped up the pressure late on.
Lawson moved dangerously into the box on 84 minutes but was brushed aside by Filip Lesniak - penalty appeals waved away - and the same defender did well to slide the ball away from Mandron a minute later. Winks then threw himself in to block from Greenwood. Andrew Nelson dragged the next half-chance wide from 14 yards.
We held firm and Harrison's hat-trick arrived in the first minute of added time, a real striker's finish. Joe Pritchard poked the ball his way and Harrison didn't think twice, drilling low across Stryjek into the far corner.
Spurs U21s: Glover, Walker-Peters, Lesniak, Walkes, Georgiou (Maghoma 90), Stylianides, Winks, Miller, Goddard (J Pritchard 69), Sonupe (Marsh 82), Harrison. Substitutes (not used): Roles, McDermott (GK).
Sunderland U21s: Stryjek, J Robson (Nelson 85), T Robson, Agnew (c), Brady, Beadling, Lawson, Smith (Wright 75), Mandron, Honeyman, Greenwood. Substitutes (not used): Molyneux, Ledger, Talbot (GK).