Harrop, who stormed past his man and fired his shot home.
Just before the break, Boro's Junior Mondal stole through on goal only to be thwarted by the excellent Henderson. It was an important intervention as, soon after the break, United made it four.
Croskery was again involved, threading a well-timed reverse ball through to Mitchell, who was brought down by Middlesbrough skipper Tinkler in the box. Mitchell stepped up to take the spot-kick and, although his initial effort was saved, he dispatched the rebound to give United an unassailable advantage.
The Reds' impressive attacking intent was again rewarded when clever play from Riley enabled the sprightly defender to race on toward the byline. United's no.2 then played a perfectly-weighted cross for Redmond to execute a delightful dink over Boro keeper Jonathan Elm. 5-0 to United.
Although there was still time for a consolation strike for Middlesbrough via Robbie Tinkler's header, the winning margin remained a handsome one. McGuinness will now hope his U18s can build on this impressive display.
United: Henderson, Riley, Barber (Thompson 58), Borthwick-Jackson, Ioannou, Goss, Harrop, Willock (Rashford 45), Croskery, Redmond (Tuanzebe 75), Mitchell.
Subs not used: J Pereira, Tuanzebe.