Couzens, of the Zappi Racing Team, blew the field apart on the second of three 20.36 kilometre circuits around the village of Grosmont as he roared away from a breakaway and stayed out in front until the finish line of the 117km Grand Prix.
His dominance was such that Couzens finished one minute 37 seconds ahead of second-placed Oscar Onley, of the Spokes Racing Team, and Thomas Gloag, from the VC Londres team.
In winning the race, Couzens boosted himself up the order of the HSBC British Cycling Junior Road Series standings with just two races to come before it ends at the end of next month.
A beaming Couzens, from Bicester near Oxford, said: “We came onto the finish circuits with three laps to go and there were still quite a lot of people there in the bunch.
“A few attacks went and nobody was really chasing. So with two laps to go at the top of the climb, I went off the front and three of us went across.
“So even in the break, nobody was working so after about five kilometres, I pushed on to see what happened and I just kept pushing on.”
Couzens was 29th in the overall Series classification before Sunday’s race and will climb up the rankings although will not come close to leader Leo Hayter who was not among the starters on Sunday, having not done many of the rounds.
It was a rare victory for him too. Couzens added: “I haven’t really got many wins so I will take this one. It is my first national podium for a road race.”
Two of the Wales team juniors finished well, with seventh-placed Iwan Evans and Sam Beckett, in ninth, sandwiching Lewis Askey, who won the Junior Paris-Roubaix classic road race in 2018 and rides for the team founded by Pontyclun Tour de France stage winner Magnus Backstedt.
Result. Cwmtillery Glass Monmouthshire Junior Grand Prix. 1. Tom Couzens (Zappi Racing Team) 3.08.49 hours; 2. Oscar Onley (Spokes Racing Team) @1.37mins; 3. Thomas Gloag (VC Londres) @2.23mins; 4. Oliver Rees (HMT Hospitals Giant Cycling Team) @2.31mins; 5. Adam Scarffe (Equiom IOM Junior Cycling Team) @2.45mins.
Notes to Editors