The 19-year-old former European youth silver medallist came on strong in the final half of an absorbing contest to secure a clear win and provide the ideal end to his impressive 2016.
Smith, a member of the GB podium potential squad, put on a fine show on his World Series of Boxing debut in Cuba in June, then got the chance to witness the preparations of this summer’s Rio Olympics at first hand.
Smith said: “I got the chance to go to Rio as a training partner and watching all the lads compete out there showed me what it takes to be an Olympian, and I am going to take that towards Tokyo.
“Going to the Olympics is a dream for me and when I see guys like Anthony Joshua, who came from nothing four or five years ago to become Olympic and world champion, that gives me even more motivation.”
Smith poured on the style to defeat the game McDonagh, who took the fight at short notice following a previous withdrawal, even resorting to some showboating in the final minute to the delight of his home city fans.
He added: “At times I tried too hard to impress the crowd but I’m still learning. I’m 19 years old and some people thought I’d been thrown in at the deep end but I’ve proved that I belong with the senior boys.”
Other winners at the GB Amateur Boxing Championships at the English Institute for Sport were Montrose middleweight John Docherty and Boris Crighton. Carl Fail edged GB podium potential boxer Connor Loftus of Leeds in a keenly contested welterweight contest.
Ramla Ali, who edged Stephanie Kernachan in a women’s flyweight bout, and rising star Crystal Barker defeated Lynn Calder by unanimous decision.