An Olympic gold medallist in Athens 13 years ago, Harrison has vowed to restore his fading reputation after high-profile defeats to David Haye and David Price, who surrendered his unbeaten record to Tony Thompson earlier on Sunday night.
Harrison unloaded a perfect left hook to knock a bloody Rossy down towards the end of the first round and the New York fighter could barely stand through the second before the referee called a timely end to proceedings at the Bethnal Green tournament.
Harrison has struggled to recover his Olympic form for nearly a decade after missing a year of boxing due to a hand ligament injury suffered in 2004.
But on Sunday night he told Sky Sports 1: "I'm feeling great. It's been nine years in the wilderness without my mojo and as you can see I've got my mojo back.
"In 2012 I fell back in love with boxing and I just wanted to get back to being my best. I left all my demons in the box and I'm really optimistic about the future, finally.
"The only thing about certainty is that everything is uncertain. Setbacks just pave the way for comebacks.
"While I'm at this level, I still think I've got something to give to boxing."
Harrison's semi-final meeting with Martin Rogan of Belfast was certainly an improvement on the Wembley fighter's opening bout, in which Denmark's Claus Bertino walked on to a sharp left hook inside 25 seconds.
Harrison, who revelled in his renewed "sharpness", immediately turned his thoughts to Rogan who struggled to land a punch on the long-limbed 41-year-old.
Rogan made several mistakes in swiping at his opponent and Harrison promptly wore him down with a succession of solid counters, with all three judges scoring a 30-27 win for the home favourite.
Harrison said: "Rogan's one of the toughest guys out there and I hit him with some shots that would have put most other guys down."
The winner of the inaugural Prizefighter tournament in 2008, Rogan had navigated his quarter-final without too much trouble as a wobbly Albert Sosnowski tumbled out of the ring in the middle of the second round.
A succession of big shots had the Polish fighter reeling early on and he eventually dived through the ropes after swinging and missing with a wild left hook, before referee Ian John-Lewis decided to stop the fight midway through the third round.
An amused Rogan said: "I think he was looking for a takeaway so I'll get him one later."
American Rossy made his experience count to book his own place in the final but there was considerable doubt over the outcome of a split decision as Britain's Ian Lewison dominated a three-round tear-up.
Lewison has had a stop-start career since turning professional in 2009 but after promising to "lay out" his semi-final opponent he kept Rossy against the ropes for nine minutes.
And a weary Rossy was sporting a black-eye by the time the judges surprisingly voted him into the final despite his absorption of most of the big shots until a late, hard rally in the third round.
Brixton-born Lewison had dual reason to be disappointed having already provided one of the shocks of the night by beating German powerhouse Timo Hoffmann on a technical knockout in the first round of the third quarter final.
The former European champion soon slumped to his feet under a volley of punches and Lewison was named the winner with Hoffmann initially unsure why the referee had stepped in.
Rossy, a decorated defensive end in his high school days, had made the semi-finals by beating compatriot Travis Walker on a split decision at the end of an attritional three-round slog.
Sunday night also saw the professional debut of Team GB boxer Thomas Stalker, who abandoned his headguard to challenge journeyman Kristian Leight before the final.
The light welterweight joined Eddie Hearn's Matchroom stable last month and the 28-year-old coped comfortably with the step up to beat Leight on points over four rounds.
"I'm always a critic of my own performance and I think it was a bit flat to be honest," the Liverpudlian said.
"I don't want to change my style too much I'm just trying to sync my punches a bit better."