Academy: Watford 2-1 Wimbledon
Rosenthal had Dave Hughes' side in front after just 10 minutes when he controlled Alex Jakubiak's pass before rifling past Wimbledon 'keeper Billy Bishop at his near post.
The equaliser was not far away, the Dons profiting from a huge slice of luck.
Goalkeeper Daniel Wilks could only clear Alfie Young's backpass into the path of the onrushing Danny Gallagher, who charged the ball down and all the way into the Hornet's net.
Watford fought hard and saw chances to waste through Jakubiak, Chris Dillon and Harry Kyprianou before Johnson struck.
From Ryan Hope's left-wing free-kick, Johnson got between the pack and 'keeper Bishop to bundle the ball over the line and get his side off to a winning start.
Watford: Wilks; Young, Johnson, Doherty; Barnum-Bobb, Dillon (Cumberbatch, 65), Stewart (Lewis, 50), Hope, Kyprianou; Jakubiak (Bawling, 70) & Rosenthal.
Wimbledon: Bishop; Kaja, Wilson, Switon, Sweeney, Vlicktink, Frost, Pearce, Oakley, Basker & Gallagher.
Highlights from Watford's official YouTube channel.